Just so we are clear the corporate directory is just not working at one branch? I am trying to figure out if the problem lies at one location or to troubleshoot the call manager corporate directory as a whole.
Assuming it is just the one branch let's start with some basics here. Can you go to enterprise parameters and let us know what the direcotries URL is, I am curious whether or not you resolve by server name (FQDN or not) or just IP Address.
Secondly, from the branch that is having problems can you try the directories URL and from a PC Browser see if you can view that page at all.