Weird VPN situation.

I have a situation with a new remote site I just set up. We have a Site to Site VPN with Site A Win2k Server, and ISA. Site B Win2003 Server, with ISA. The Site to Site VPN works just fine. My problem is with the clients at site A, all of which are XP Pro. For some strange reason the clients dont take advantage of the Site to Site VPN. I dont know if its a routing thing, or a NIC configuration, or ISA problem. Each XP Client can connect with a VPN initiated from the local workstation, but cant see any remote resources without it. Any suggestions would be great.