Damper problem

I live in a 3000 sq ft house and the builder put in only one a/c unit with a damper system. The master bedroom gets no a/c (or heat). I went in the attic and found that the master bedroom has its own duct. Everything looks brand new. No leaks at all.

The damper system seems to be working, but I wouldn't know really. the lights turn on and off when it cycles. My best guess is that there is an obstruction in the master bedroom duct. Possibly the damper is not working.

Guess I'm just looking for advice on how to proceed. I'm a home remodeler, so I'm not a complete idiot when it comes to this stuff, though I am not HVAC certified. I'm a bit in over my head with this one and would like not to have to call an a/c guy out to tell me I need to add another a/c. (Duh)

I rent, and i doubt the owners are going to pay 10k+ for a new a/c. I'd at least like to be able to tell the a/c guy what to do when he gets here so that he doesn't find 20 other things to do before fixing the problem.