Hi,
There can be different reasons.
Make sure Firewall exceptions are in placed if windows firewall is used.
Following articles can help you to troubleshoot the issue.
Testing Communication from an Endpoint Protection client to the Endpoint Protection Manager
http://www.symantec.com/docs/TECH102682
Symantec Endpoint Protection Manager 12.1 Communication Troubleshooting
http://www.symantec.com/docs/TECH160964
Client is not appearing in Symantec Endpoint Protection Manager (SEPM), error: HTTP returns status code=407