Does she stop when you sit down in your seat? If she does, try and channel this. Try and sit deeper in your seat, relax and slow your body down. Don't ask for a stop, but almost. If she wants to stop, squeeze her on so she keeps walking forward. You could do this when working on the serpentines. In the middle, you could ask her to one rein stop, bend to you, disengage her hindquarters, then go foraward.
Another thing you could do would be to be walking down the rail, then bring her to stop at a 45 degree angle to the wall. Have her stop, back up a few steps and pivot so she is facing the opposite direction, then walk off. This should get her using her hind end.