SEP 12.1 RU1 MP1 - Scan Logs indicate scan never completes

Hi - 

I am looking for a solution for the following problem:

When new definitions arrive, per our policy, a virus scan kicks off as expected (DefWatch Scan).  For ~20 out of 60 servers, the scans take over 8 hours to complete, despite being tuned for best scan performance.  Also, the Scan log (console) and local scan log (host) indicate that the scan is still taking place.  It seems as if the agent never registers the scan completed and doesnt log it, or the scan never completes.  Not sure.

Does anyone have any ideas?  Thanks.