What other jobs are running on that server? I had a similar problem a while back and found that another job, scheduled to start 5 minutes before mine, ran long causing the agent to 'skip' my job. I know servers are supposed to be able to multitask, but just for grins and giggles I rescheduled my job for 5 minutes before the other job's scheduled start time. That seemed to work like a charm - never had another 'skip'!