Check if they are reporting to different domain within SEPM
click on admin - servers - domains, Do you see another domain other than Default if yes, then select administer then search for clients
Or
These are cloned machines, hence only one machine will report, others will not
Duplicate Hardware IDs result in only one client showing up in the Symantec Endpoint Protection Manager for multiple systems
https://support.symantec.com/en_US/article.TECH97626.html