Hello,
Could you please provide more details?
What OS are you running the SEPM on?
What version of SEPM 11.x and SEP 11.x clients are you running?
When did this issue started occurying?
How many clients are not reporting?
Do you have a firewall on the server / clients?
Here are few Troubleshooting Articles:
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
Could you please upload us the sylink.log from 1 of the client machine which is having an issue, which would assist us understanding the root cause of the issue. Check this Article below on how to collect the Sylink.log
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!!