Domain not showing up in Net Nhood

We have a WAN with 5 sites. All connected by frame with internal ip numbers. From one site(E), I can see all others in Net Nhood. From other sites (A-D), we can’t see that site. Router is okay, same as other sites except ip numbs (checked out by vendor & Cisco). Must be a Windows 2000 server thing… Can map, ping, traceroute, VNC, and other wise use the site (E) remotely. Can’t use Active Directory for E from other sites A – D. This has stumped “experts”.