This is the wrong forum (this is for ticketing software), but check to make sure the client can communicate with the server. Since one client works and the other doesn't, it doesn't seem to be the server's problem. You may want to try disabling Windows Firewall on the client that's having trouble, and confirm that you can contact your SEPM on TCP port 8014, which is the default.
If the SEP icon on the trouble client has no green circle, it is either unmanaged or is managed but cannot communicate with the server. If it has a green circle, it is managed and communicating with the server.
If a client is "not updated," I assume you mean definitions are out of date. But using the Update Policy command will not update content. This option is for updating the configuration by requesting a new policy from the server, if a new policy exists. If you want to update content, click the Fix button within the GUI.
The reason you might be able to install SEP but not communicate afterward is that different ports are used for installation (139 and 445, I think) than are used for communication (8014).