Hello,
What verison of SEP 11.x are you running?
Secondly, What OS are you running the SEPM on?
In case if the Server is Windows 2003, then make sure you have Either the Windows Firewall turned off OR
make necessary port exceptions.
Secondly, if the Server is Windows 2008, make sure you have the UAC turned off and the Windows Firewall turned off OR make necessary port exceptions.
Secondly, could you pull Sylink.log from 1 of the client machine and upload it to us to understand the root cause of the issue. To pull the sylink.log, follow the article below:
How to enable Sylink debugging for the Symantec Endpoint Protection 11.x and 12.1 client in the Windows Registry
http://www.symantec.com/docs/TECH104758
Hope that helps!!