There are a number of different issues that can create problems with Active Directory:

When someone has troubles and unclear as to why I usually suggest the following:
Run diagnostics against your Active Directory domain.

If you don't have the support tools installed, install them from your server install disk.
d:\support\tools\setup.exe

Run dcdiag, netdiag and repadmin in verbose mode.
-> DCDIAG /V /C /D /E /s:yourdcname > c:\dcdiag.log
-> netdiag.exe /v > c:\netdiag.log (On each dc)
-> repadmin.exe /showrepl dc* /verbose /all /intersite > c:\repl.txt
-> dnslint /ad /s "ip address of your dc"

**Note: Using the /E switch in dcdiag will run diagnostics against ALL dc's
in the forest. If you have significant numbers of DC's this test could
generate significant detail and take a long time. You also want to take
into account slow links to dc's will also add to the testing time.

If you download a gui script I wrote it should be simple to set and run
(DCDiag and NetDiag). It also has the option to run individual tests
without having to learn all the switch options. The details will be output
in notepad text files that pop up automagically.

The script is located on my website at
http://www.pbbergs.com/windows/downloads.htm

Just select both dcdiag and netdiag make sure verbose is set. (Leave the
default settings for dcdiag as set when selected)

When complete search for fail, error and warning messages.

Description and download for dnslint
http://support.microsoft.com/kb/321045