And you assigned the same LiveUpdate policy to the clients that should be receiving content from the GUP?
It may take a little time for the GUP to get all the necessary content updates for all your clients. How long has it been?
On the GUP, check the SharedUpdates folder. This contains all the content updates clients will receive. Should be located in the SEP folder under Program Files.
Here are a couple helpful articles on troubleshooting GUPs:
Troubleshooting the Group Update Provider (GUP) in Symantec Endpoint Protection (SEP)
https://www.symantec.com/business/support/index?page=content&id=TECH104539
How to confirm if SEP Clients are receiving LiveUpdate content from Group Update Providers (GUPs)
https://www.symantec.com/business/support/index?page=content&id=TECH97190