Make sure that in Config - System - Servers - Authentication, when you add the PDC/BDC in as the auth server, enter the PDC/BDC's server name where it asks for "Domain Controller Name", don't enter the domain name here.
Does it work when you use the Test button on the Auth server screen, and not work when a user tries to come in? Make sure in the user group under the IPSec tab you have NT Domain listed as the Authentication Type.
Then go to Config - System - Events - Classes and add the AUTH and AUTHDBG classes at Severity to Log of 1-13. Go to Monitoring - filterable Event Log, hit Clear Log, try a test authentication then hit Get Log, then check to see what's going on. Post the output back here if you're not sure what's happening.
