Since so many different factors depend on the brokering process, this is a really helpful article that covers troubleshooting just about every angle of the XenDesktop Brokering process. Are your desktops not registering with the DDC? Check this article here (although a little outdated):
http://blogs.citrix.com/2012/07/23/troubleshooting-xendesktop-brokering-process-2/