Hello,
Could you please let us know which version of SEP 11.x are you running?
Secondly, Did you check if the SEPM is updated with the Definitions?
Is this issue occurying on all client machines OR few of the Client machines?
For Troubleshooting, check these Articles:
Symantec Endpoint Protection: LiveUpdate Troubleshooting Flowchart
http://www.symantec.com/docs/TECH95790
Symantec Endpoint Protection: Troubleshooting Client/Server Connectivity
http://www.symantec.com/docs/TECH105894
Hope that helps!!