Hello,
Are the clients actually showing offline, with regards to SEPM communication? (Help and Support (or Help if 12.1) > Troubleshooting)?
If they are 'offline', and they don't have a LiveUpdate schedule set (or don't have internet access), then that's why the definitions are getting out of date. It may not have anything at all to do with definition corruption.
If it does say 'offline', enabling Sylink debug logging is the best way to track down why communication is failing with the SEPM. (With SEP 12.1, Tamper Protection must be disabled first.)
How to enable Sylink Debugging for Symantec Endpoint Protection in the registry
http://www.symantec.com/docs/TECH104758
Symantec Endpoint Protection Manager 12.1 Communication Troubleshooting
http://www.symantec.com/docs/TECH160964
Symantec Endpoint Protection: LiveUpdate Troubleshooting Flowchart
In case of Corrupt definitions, you may have either do that Manually.
Incase, of SEP 11.x, check this Article:
How to clear out corrupted definitions for a Symantec Endpoint Protection client manually
http://www.symantec.com/docs/TECH103176
Incase, of SEP 12.1, check this Article:
How to clear out definitions for a Symantec Endpoint Protection 12.1 client manually
http://www.symantec.com/docs/HOWTO59193
Hope that helps!!