1st - Check log output on client and NS Server side:
- Client side logs: C:\ProgramData\Symantec\Symantec Agent\Logs
- NS server side logs: C:\ProgramData\Symantec\SMP\Logs
Maybe there is a communication problem via HTTPs, since required certificate isn't in trusted root, etc..
2nd - Check whether this client is in filter on NS server or not:
3rd - Make sure that registration rule is applicable for this client to be successfully registered with NS server:
4th - You can try to reset a GUID for this problematic managed endpoint
- Execute this cmd on client computer: "C:\Program Files\Altiris\Altiris Agent\AeXAgentUtil.exe" /ResetGuid