You need to check if the clients are in network
take a client from the list and do a remote desktop; check if that reporting to the manager ( u should see green dot on the taskbar with symantec endpoint installed)
once thats done ; open help and support of the client interface and check the group its reporting too.
the client might be in user mode
in sepm
under clients - select the display filter, check offline and user mode clients
do a search ; you should be able to see the clients.