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Calendar sync failures can appear as missing appointments, events that do not update after changes, or a mobile device that stops reflecting calendar entries entirely. The most common causes are a disrupted Exchange or Microsoft 365 connection, an account that needs to be re-added on a mobile device, stale cached data, or insufficient permissions on a shared calendar. Work through each section below in sequence until your calendar is syncing correctly.
These steps cover Microsoft 365, Exchange Online, and Exchange On-Premises environments. If your organization uses Google Workspace, the account re-addition steps and connectivity checks still apply, but the specific settings will differ.

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1. Check Exchange / Microsoft 365 Connectivity in Outlook

Before investigating device or account-specific settings, confirm that Outlook itself has a healthy connection to the mail server.
1

Check the connection status indicator

Look at the bottom status bar of the Outlook window. It should read Connected to: Microsoft Exchange or Connected. If it shows Disconnected, Trying to connect, or Working Offline, your connection to the server is interrupted.
2

Disable Work Offline mode

Click Send/Receive in the ribbon and check whether Work Offline is highlighted (active). If it is, click it to toggle it off and reconnect.
3

Run Microsoft Support and Recovery Assistant

For Microsoft 365 accounts, Microsoft’s official diagnostic tool can detect and fix connectivity and sync issues automatically:
Run the tool and follow the prompts to diagnose your Outlook profile.
4

Check Microsoft 365 service health

Visit the Microsoft 365 Service Health Dashboard or ask your administrator to check it. A widespread Exchange Online outage will affect all users and cannot be resolved locally:
5

Test Exchange connectivity with Outlook's built-in tool

Hold Ctrl and right-click the Outlook icon in the system tray, then select Test E-mail AutoConfiguration. Enter your email address, uncheck Use Guessmart and Secure Guessmart Authentication, then click Test. Review the Results tab for any errors in the Autodiscover or Exchange connectivity process.
If the AutoConfiguration test shows an Autodiscover failure, your IT administrator may need to verify the Autodiscover DNS record or internal Exchange URL configuration.

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2. Force a Calendar Sync in Outlook

Sometimes the Outlook client simply needs a nudge to pull the latest calendar data from the server.
1

Trigger a full Send/Receive cycle

Press F9 or click Send/Receive → Send/Receive All Folders. This forces Outlook to synchronize all folders, including Calendar, with the server.
2

Manually sync the Calendar folder

In the folder list on the left, right-click your Calendar folder and select Properties. Under the Synchronization tab (available for Exchange accounts), click Clear Offline Items, then click OK. Outlook will re-download all calendar data from the server.
3

Rebuild the local OST cache

If the above does not resolve the issue, close Outlook and navigate to:
Rename your .ost file (e.g., append .old to the filename) rather than deleting it. Outlook will create a fresh OST file and re-sync all data — including your calendar — from the server on next launch.
Renaming the OST file triggers a full re-download of your mailbox. This can take several minutes to hours depending on mailbox size and connection speed. Do not perform this step immediately before a meeting or deadline.

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3. Fix Calendar Sync on a Mobile Device

Mobile calendar sync issues are most often caused by an account in an error state. The fastest resolution is to remove and re-add the account.
1

Check the sync error notification

On iOS: Go to Settings → Mail → Accounts, tap your Exchange or Microsoft 365 account, and look for a warning icon or error message. On Android: Go to Settings → Accounts, select your account, and check for a sync error badge.
2

Force a manual sync

On iOS: Open the Calendar app, pull down on the calendar view to trigger a refresh. On Android: Open Settings → Accounts → [Your Account] → Sync Now.
3

Toggle calendar sync off and on

On iOS: Go to Settings → Mail → Accounts → [Your Account] and toggle Calendars off, wait 10 seconds, then toggle it back on. On Android: Go to Settings → Accounts → [Your Account] and uncheck then re-check Sync Calendar.
4

Remove and re-add the account

If toggling sync did not help, remove the account entirely and add it back fresh:iOS:
  1. Go to Settings → Mail → Accounts → [Your Account] → Delete Account.
  2. Return to Accounts → Add Account → Microsoft Exchange.
  3. Enter your email address and tap Next. Sign in with your credentials when prompted.
  4. Ensure Calendars is enabled and tap Save.
Android (Outlook app):
  1. Open the Outlook app, tap your profile picture, then Settings → [Your Account] → Delete Account.
  2. Re-add the account via Settings → Add Account → Add Email Account.
5

Check the Exchange server address (legacy ActiveSync)

If you are using the native iOS/Android mail app with ActiveSync rather than the Outlook app, ensure the Exchange server address is correct:
The Outlook mobile app (iOS and Android) is generally more reliable for Microsoft 365 and Exchange calendar sync than the built-in device mail clients. If you continue to experience issues with the native mail app, try switching to the Outlook app.

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4. Check Shared Calendar Permissions

If a shared calendar (a colleague’s calendar, a room calendar, or a team calendar) is not showing or not updating, the issue is likely a permissions problem rather than a connectivity problem.
1

Verify your permission level

Ask the calendar owner to confirm your access level. In Outlook, the owner can check by right-clicking the calendar, selecting Properties → Permissions, and reviewing your entry. The minimum required for basic viewing is Reviewer.
2

Re-accept the sharing invitation

Ask the calendar owner to resend the sharing invitation. Open the invitation email and click Open this Calendar (or Accept). This refreshes the permission link on your client.
3

Remove and re-add the shared calendar

In Outlook, right-click the shared calendar in the folder list and select Delete Calendar. Then re-add it:
  1. Go to Home → Open Calendar → Open Shared Calendar.
  2. Type the name or email address of the calendar owner and click OK.
4

Check calendar overlay mode

If the shared calendar appears in your list but events are not visible, ensure it is not toggled off. Click the calendar name’s checkbox in the My Calendars or Other People’s Calendars panel to ensure it is enabled (coloured, not greyed out).
5

Verify Microsoft 365 admin permissions (for admins)

For administrators, verify calendar delegation using PowerShell:
Default calendar sharing in Microsoft 365 allows users to share their calendar with permission levels ranging from Free/Busy only to Full Details to Can edit. If a user can see events but not their details, they likely only have Free/Busy access — ask the owner to increase the permission level.

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Escalation

Escalate if any of the following apply after completing all steps above:
  • The Outlook AutoConfiguration test shows Autodiscover failures that persist after reconnecting, indicating a DNS or Exchange configuration problem.
  • Multiple users in your organization are experiencing calendar sync failures simultaneously — this suggests a server-side or Exchange/Microsoft 365 issue.
  • Re-adding the account on mobile fails at the sign-in step, which may indicate a Conditional Access policy, MFA configuration, or licensing problem.
  • Calendar events from a specific date range are permanently missing after an OST rebuild, which may require mailbox recovery.
  • Shared calendar permissions appear correctly configured but sync still fails — this can indicate a broken calendar delegation object that must be repaired by an Exchange administrator.
When logging a ticket, include:
  • Whether the issue affects your own calendar, a shared calendar, or both.
  • The devices and Outlook versions affected.
  • The error message shown in the mobile account settings, if any.
  • The results of the Outlook AutoConfiguration test (screenshot of the Results tab).