WSUS Ports and or Protocols being blocked?

We are moving from SUS to WSUS. I've got everything set up and working OK.  However, a number of our PCs actually live on a partner network. We have connectivity to them via a firewall. We use their IP addresses, but our DNS and WINS servers. The users authenticate to our domain (AD native) and things like port 80 traffic and email work OK. These PCs are not showing up in the list of computers that WSUS knows about. I have modified the GPO that we use for patch management and I have checked the registry settings fo a couple of them and I can see where the GPO has been set correctly.  My assumption is that they are inadvertantly blocking some port or protocol necessary for the proper connectivity with the WSUS server. Does anyone have any thoughts on this?

Thanx...