WSUS Clients get updates from WSUS server, but they are not showing up on WSUS console

I have large number of clients getting patches from the WSUS server, but  some are not showing up on the WSUS console. I only have one WSUS server running from Windows 2008 R2 server. Most clients I noticed  not showing up are Windows 2008 servers. When I check the WindowsUpdate.log file on those clients, everything looks normal. I see that Server URL is correct, downloading patches and installing patches. On WSUS server, I checked logs from inetpub\logs\logfiles\W3SVC1 and they all reports fine. On client's registry, WindowsUpdate is configured correctly.

I configure the Windows update from a Group Policy, so it's not a mis-configurations. On the console, I do see over 1100 clients reported.

Where can I start troubleshoot why they don't show up on the console?

Thanks,

Myung