Sometimes builds will get stuck in TFS 2010 and no build with a priority of "Normal" will build. You'll find that you have to change to priority to something higher in order to actually trigger the build. This usually happens because of an unhandled build failure on a build with "Normal" priority. TFS thinks there is another "Normal" build ahead in the queue, and therefore holds off on building your new request.
So how do you fix it? This requires going into the database. Open that database for the collection that is having the problem and look in the tbl_BuildQueue table. Check the Status column in this table. Anything with a "2" means it is still queued (and therefore blocking any new builds). Change these to a "16", which means "Cancelled". That should fix your problem!