One possible cause for this, is that the scheduling plan in place, may not be optimally configured.
Especially in large environments, pay special attention to the following:
Having a Start Time at 09:00AM and an End Time of 09:30AM may not be enough time to allow jobs to complete. This is especially important in larger environments, where you may have quite a large number of agents all starting their backup at the same time.
Depending on the amount of simultaneous transfers configured on your core, the majority of these will join a queue.
If the End-Time configured in the schedule only allows a 30 minute window for the backup to run, then a lot of these jobs will simply not run, because they never leave the queue.
To avoid this situation please do the following:
Group 1: (Critical agents) Backup 24x7 with 1 hour intervals, starting at 12:00AM
Group 2: ( Non-Critical) Backup 24x7, with 3 hour intervals, starting at 12:30AM