The AD delta scan is known to lock up and fail to exit. If this happens you can clear it by cancelling the schedule. However, we recommend that you remove the delta flag from the AD scan so it will do full scans only, which still only take a few minutes. This will resolve the reliability of that schedule, as there are known issues in the .NET API for DirSync control used by the delta scan.
You can also add schedule health probes which will trigger alerts when a schedule is stuck.