WUS does not push down to clients

I have all the computers in my network with a WUS group policy. I have a building which me server is in, with some of the clients, and a remote location with the remaining clients.

On a recent visit, I discovered that several of the computers are NOT updated with all the patches, hotfixes they should have. The clients in the same building as my server, however, updated the same day they were installed.

I tried to gpupdate /force, but that doesn't seem to have triggered the updates to be downloaded.

Where should I be looking for problems? I see three places where something could go wrong: AD and the server, the client itself, or something in the communications between the client and the server. Where would you check first?