Are you giving her a balancing half halt before asking for the transition? You might just be asking too suddenly and she's being caught off guard. Make sure before you start the transition, sit tall, weight your seat bones evenly and momentarly close your outside rein to balance her back on her hind end. Then ask for the transition. Also, before you do the downward transition, is she traveling round or hollow? If she's not balanced before you ask for a change, she won't have a remote chance of balancing for the transition. Good luck.