5 computers, same wrkgrp, can't see each other

I have two Win2k3, one XP, one Win2k pro, and one Win2k server. All are in
the same workgroup (called buddy01). All are the same subnet (192.168.0.x).
Really strange:

Win2k3(a) can see all the computers. Win2k3(b) can only see the XP. The XP
can't see any computers. The Win2kpro can see the Win2k3(b) only. The
Win2kserver can see XP and Win2kpro.

How on earth is this possible when every computer is in the same workgroup
and the same subnet? Am I missing something in my networking skills? All
these computers are wired through a switch. No router, no gateway - they
don't require the Internet. Any idea's??