Hello,
What OS are the clients in question running?
Make sure the UAC and Windows Firewall Services are stopped and Disabled.
Could you please upload us the Sylink.log from the client machine, so that we could understand the root cause of the issue.
Check the Article below on 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
Are these client machines imaged / cloned machines?
If yes, sounds like it could be a possible duplicate HWID issue. I would follow the steps in the document below to identify any clients that my share a HWID.
http://www.symantec.com/docs/TECH163349
Hope that helps!!