Bonus rooms over garages can be notorious for thermal envelope deficiencies. It's as if whoever builds them thinks the space is called "bogus room" vs. "bonus room".
Things you can check as a homeowner...Dan already hit on a few...but check the walls of the bonus room to see if any are adjacent to attic space. These are known as "knee walls". Check the insulation on walls like these. Check insulation over ceiling of bonus room, if you can get to it (if you can't, it will make you wonder if any is up there at all, since if you can't get to it, who else could to install the insulation?). You can also get an infrared thermometer and go around the walls, floors, and ceilings within the bonus room during a hot day with the a/c running in the bonus room. You'll find the hot spots quickly with that tool, and you'll have a basic idea where your insulation and thermal bypass problems are.
Psychrometrics: the very foundation of HVAC. A comfort troubleshooter's best friend.