She may be hollowing because you're bareback. When i ride my boy around the pasture bareback he tends to hollow his back more than undersaddle. In the saddle i can get off of his back easier and get better collection. My appy at one point refused to canter in the ring, he would canter on the trails behind other horses and alone but wouldn't canter in the ring. I at one point got so frustrated that we did a fast trot for a few sessions while urging him to go faster, he eventually figured out that cantering was easier than going around the ring at a fast trot. We perfected the transition later when he understood what i wanted.