Have you recently upgraded the SEPM? Because there is a known issue with SEP 12.1 RU5.
See this: After upgrading SEPM to 12.1 RU5, 32-bit virus definitions do not update
Fix ID: 3671931
Symptom: After a Symantec Endpoint Protection Manager upgrade to 12.1 RU5, the 32-bit Virus and Spyware definitions are not up to date.
Solution: Gave the semsrv and semwebsrv accounts full access to the LiveUpdate symcData folder.
Reference: http://www.symantec.com/docs/TECH230558
OR
Try to repair the SEPM & update using .JDB files to see if it makes any difference.