By default, Retrospect Client will bind to the first available active IP address. In a dual-NIC setup (e.g. network card and wireless in a laptop) that "first available" can vary.
So you'll need to tell the client which IP to bind to -- see this page for details.
Obviously, only works for static IPs. Luckily it sounds like that's what you are using.
That might be enough to get you going. Otherwise, could you re-post the problem but refer to machines as "clients" for the ones you want to back up and "server" for the one doing the RS backup? Like David, I'm getting a bit confused as to what's what!