This issue is typically due to a synchronization error between the Outlook mobile application and the Office 365 server, which is quite easy to fix:
- Open the Outlook Mobile app on your device, and click on the "three stacked bars" icon in the top, left-hand-corner. This will bring up your folders menu.
- At the bottom you should see a 'gear' icon, which will take you to the settings page.
- Under accounts, tap your email address and the app will take you to an Account Info screen.
- Select the Reset Account option at the bottom.
This will force Outlook to re-sync and re-connect everything from your device, and establish a new connection to the server.