1C entry gives a 0.0.0.0

When you do a NBTSTAT -c on a workstation it comes back with 0.0.0.0 for 1c. Dialing in over Ras sometimes gives the user the error domain controller cannot be found. Then when they try agian it works. Lhmost works for the dialing machines, but whatis causing 0.0.0.0 on the lan workstations, any direction point would be appreicated. I have a net mon coming to me via the customer.