1. Check the placements of your calendars in the Calendar app or Outlook.com calendar
2. Verify that the Contacts, Tasks, and Notes options are activated in the calendar app or Outlook.com calendar
3. Verify that the correct calendar group is selected as the default calendar
4. Verify that the calendar sync feature is enabled in the Calendar app or Outlook.com calendar
5. Check that the calendar in question is marked as active in the Settings.