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System is not receiving updates, verify the policy serial number is correct on both SEPM and SEP clients.
If definitions are also not updated, go through the following article.
How to determine if virus definitions of Symantec Endpoint Protection client (SEP) 11 or 12 Small Business Edition, are corrupted
http://www.symantec.com/docs/TECH97677
How to clear out definitions for a Symantec Endpoint Protection 12.1 client manually
http://www.symantec.com/business/support/index?page=content&id=HOWTO59193
Can try to repair the SEP client through add/remove programs.