Check to make sure your mobile device is connected to the Exchange server.

1. Check to make sure your account information is correct.
2. Verify that your device has the latest operating system.
3. Ensure that you are on the correct wireless network.
4. Verify that both the device and the calendar are set to the correct time.
5. Clear the caches of your browser, Exchange ActiveSync, and calendar.
6. Try signing in to an alternate account.