Logging on to Domain

I am having problems logging Windows 2000 an xp pc’s on to our domain in some of our locations on our WAN.
We have a Windows 2000 server pdc with a Windows Nt bdc in our headquarters, all our other locations have nt servers.
The problem is in two locations I cannot log new pcs onto the domain. I can only map drives and log the users on locally to the pc.
Any help on how to overcome this problem would be appreciated.