did you install MxMonitor / moni ?
sometimes by default moni has stupid ignore by speeds set as default
what is actually happening with these users that 'can't connect'
are they timing out on incoming connection ? etc
in winmx settings 'file transfers' running / pending -- uploads /downloads -- default timeout is 45 seconds
try increasing these to 300 sec..
swing by the online winmx help room
I am sure they wont mind seeing if you can connect to them in there