This could be because the Term has not started or has already ended. No notifications will be sent out to any of the Users if the Terms have not started, or have ended because the system does not see any active enrollments for the group.

To fix this, delete the Announcements or other Content Type and re add them to the Group providing the Terms have started or have not ended.

If you need to adjust your Term start or end dates, navigate to:
  1. Core > Settings > School Information > Years and Terms 
  2. Hover over the Start or End date or the Term and click the Pencil to inline edit the Term Date