Taskflows stall

Hello, we are on 11.1.2.3.500.  We are heavy users of taskflows.  Recently, for some unknown reason, our taskflows will stop running.  The scheduler does kick off the taskflow and send it to Active status.  However, the stages never begin.  And, in the Task Audit Log, the Taskflow user ID we use doesn't show up as logged in.  The taskflows will be running and processing just fine.  Then all of a sudden, they just stop.  As I said, the scheduler continues to send them to Active status, but the stages never begin and never process and the Taskflow user doesn't seem