Hello,
What version of SEP are you running - SEP 12.1 OR SEP 11.x?
Is the SEP client installed on the client machine a managed client OR Unmanaged?
If Managed and SEP 11.x, check these Articles below:
Symantec Endpoint Protection Manager 11.x Communication Troubleshooting
http://www.symantec.com/docs/TECH102681
Symantec Endpoint Protection: Troubleshooting Client/Server Connectivity
http://www.symantec.com/docs/TECH105894
If Managed and SEP 12.1, then check these Articles below:
Symantec Endpoint Protection Manager 12.1 Communication Troubleshooting
http://www.symantec.com/docs/TECH160964
Troubleshooting communication problems between the management server and the client
http://www.symantec.com/docs/HOWTO55017
If Unmanaged, then make sure you replace the sylink.xml, follow the Article below:
Converting an unmanaged client to a managed client
http://www.symantec.com/docs/HOWTO55039
How to convert Symantec Endpoint Protection (SEP) clients from managed to unmanaged without uninstalling and reinstalling
http://www.symantec.com/docs/TECH104010
Hope that helps!!