The demotion of domain controllers can fail when the domain controller you are performing this action on has no connectivity with other domain controllers in the domain. If this happens, try selecting the Force The Removal Of This Domain Controller check box on the Credentials page of the AD DS Configuration Wizard when you are attempting to demote the domain controller.
This tip is excerpted from my latest book Training Guide: Installing and Configuring Windows Server 2012 from Microsoft Press.
Mitch Tulloch is a nine-time recipient of the Microsoft Most Valuable Professional (MVP) award and a widely recognized expert on Windows administration, deployment and virtualization. For more information see http://www.mtit.com.