Hi,
First, what is the version of your WSUS clients? Is your WSUS server patched?
http://support.microsoft.com/kb/2734608
Then check GP result, did your WSUS clients get GPO correctly?
Or manually check the registry key.
HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\WUServer HKEY_LOCAL_MACHINE\SOFTWARE\Policies\Microsoft\Windows\WindowsUpdate\WUStatusServer
Make sure the two registry key point to your WSUS correctly.
Check connectivity between WSUS server and clients. Port 8530 by default in windows server 2012, confirm your firewall allow the traffic. Ping, telnet
will help address network issue.
Hope this helps.