Bowed bars is a common problem with queen & king size quilts. Some frames even recommend slipping the original poles inside of round, metal conduit to eliminate the bowing. I had to do that for my B-Line Studio frame & it was a common problem on the older Handiquilter frames, too.
Also, your frame might need re-leveled. It may have sagged causing a "hump". Some frames seem to walk around the room and need re-leveled with each use. My B-Line did on a carpeted basement floor but I didn't have the problem until I moved it downstairs and extended it from 10' to 12'.