Outlook Calendar Not Syncing: How to Fix It

Try these in order

1. Check the calendar is switched on for that account. In the Outlook app go to Calendar, tap the three lines top-left, and make sure Calendar is ticked under your email address (Microsoft Support).
2. Make sure your account type can sync calendars. IMAP and POP accounts sync email only — calendar and contacts need Exchange ActiveSync (Microsoft Support).
3. Re-add the account with the right setup. If your address isn't listed under Calendar, delete it and add it back using Exchange ActiveSync rather than IMAP (Microsoft Support).
4. Turn on the phone permissions Outlook needs. On iPhone, Settings > Outlook > turn on Contacts and Background App Refresh; on Android, Settings > Applications > Outlook and enable Contacts (Microsoft Support).
5. Reset the account inside Outlook. Open Settings, tap the account that isn't syncing, and choose Reset Account (Microsoft Support).
6. Remove the device partnership. In Outlook on the web, Settings > General > Mobile devices, delete the device, save, restart the phone, and sync again (Microsoft Support).
7. Delete and re-add the account. If resetting doesn't work, removing and re-adding the account is Microsoft's next step (Microsoft Support).