If the back-end processor fails to process a job, the job is left in the queue. After three failed attempts to process the job, the processor gives up on the job. In an effort to detect issues earlier, the Back-end Job Processor Unity Scheduler task displays failed jobs in the Failed Jobs tab located in the Details section of the Unity Scheduler.
You can either right click the job to view the details, requeue, or purge the job, or you can select a job and click the buttons on the bottom of the tab to perform the actions.