Jobs Cancelled due to Job Engine Service Stopping

I'm constantly having issues with jobs stopping, being queued and the disk media going offline.  I am backing up to disk, and so far whenever I have a scheduled job I keep getting the error: 

The job was canceled because the response to a media request alert was Cancel, or because the alert was configured to automatically respond with Cancel, or because the Backup Exec Job Engine service was stopped.

V-79-57344-33039 - Error - Mount failed.
User canceled a Physical Volume Library operation.

However, when I re-enable the disk media and run the job manually, it works.  What's going on?