If your Google Calendar won’t sync and/or you haven’t used your ITB account in a while, then Google Calendar has lost its connection. This is a simple fix and is related to Google Calendar basically “losing connection”.
To fix this issue, simply log into your ITB account and click on your profile. Once there, you will see your Google Calendar Sync tab. Turn the sync off and then on again. Give it a few moments and your calendar should begin to sync normally again. Please see the image below to help you navigate to the correct area in your ITB account.