There could be a damper somewhere, restricting the airflow. The ducts could be very leaky and the air is pouring out into the floor joist area, or, it could just be the end of the run, not insulated, and not very effective. Is there much air movement, but it is just not the right temperature? or not air movement at all?
Sometimes, the builders are very sloppy when attaching ductwork, especially with the flexible round stuff. I had one run that had fallen out of the supply duct because they didn't use the right adapter, cut it a little short so there was tension on it, and then used duct tape to attach it rather than a tie wrap, clamp, or the metal sealing tape. Now, finding the problem can be a pain. Maybe someone with a remote control camera?