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.
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 * /verbose /all /intersite > c:\repl.txt
A quick little dump repadmin /replsum > c:\replsum.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.
When complete search for fail, error and warning messages.
Description and download for dnslint
ADReplStatus Download and Description
If you are working with folks in a forum and will be posting log errors consider using Skydrive and then include the link to the logs in the post.