If you are running previous version then this issue fixed on SEP 12.1 RU2 MP1
Clients report to the Symantec Endpoint Protection Manager as offline, even though they are online
Fix ID: 3002170
Symptom: Clients will randomly report into the Symantec Endpoint Protection Manager as offline, even though they are actively online and available.
Solution: Updated the client USN management to properly update the client status in the Symantec Endpoint Protection Manager reports.
Might be problem with Duplicate Hardware ID as well, give this a try
Duplicate Hardware IDs result in only one client showing up in the Symantec Endpoint Protection Manager for multiple systems
http://www.symantec.com/business/support/index?page=content&id=TECH97626