Hello,
Is the Symantec Endpoint Protection Manager taking updates?
Is there Internet connection running on the SEPM Server?
The Managed clients do take their definition updates from the Symantec Endpoint Protection Manager (SEPM).
The Symantec Endpoint Protection Manager (SEPM) communicates with each managed client upon the heart beat interval set and as per the Liveupdate Policy set.
Check these Articles:
Symantec Endpoint Protection : The Heartbeat Process
https://www-secure.symantec.com/connect/articles/symantec-endpoint-protection-heartbeat-process
Symantec Endpoint Protection Manager 12.1 - LiveUpdate - Policies explained
http://www.symantec.com/business/support/index?page=content&id=TECH178257
Hope that helps!!