I can't add another DC to the AD Domain. What can I check ?

Steps for fixing the problem when DCPROMO does not find the domain.
1. Verify that the existing domain controller is pointing to a Windows 2000 DNS server. Do not point it to any external ISP DNS servers.
2. Open the DNS MMC, double click forwarders so that you can see the zone for your domain.
3. Right click on this zone and select properties. Verify that your zone is set to allow dynamic updates, if not change it so that it does.
4. Double click your zone to expand it. You should have 4 subfolders (_MSDCS, _SITES, _TCP, _UDP) and a few records.
5. If the zones do not exist you should open a command prompt.
6. Type IPconfig /registerdns and enter
7. Type net stop netlogon
8. Type net start netlogon (restarting netlogon will force the service to register its SRV records with the DNS zone thus create the missing subfolders. The records that will be registered are in winnt\system32\config\netlogon.dns).
9. After restarting netlogon go back into your DNS zone and verify that you have the subfolders that were mentioned in 4. above.
10. If the folders are not there you may want to try running netdiag.exe /fix from the support tools. Or try restarting netlogon again.
11. On the DC that you are trying to promote verify that it is pointing to the Windows 2000 DNS server that we have been working on for DNS.