Thanks so much for the advice!
I don't know what you are doing or have tried so far, but I would suggest just make sure her nose is tipped to the right before you cue her. If she is confused to what you are asking her to do, make sure you give her very clear cues.
Because the horse is young, don't try to make the aid 'invisible' to spectators. Be as clear as you can be, so bend the head slightly inwards, clearly lay back the outside leg and don't let the reins go. If she also has this problem without you on her, so when lunging, she might just need a bit more muscle.
If she does not get the change behind, most likely she is lacking muscle. I would try riding her up lots of hills at the trot
Try starting on a circle and trotting very collected. Ask very clearly for a canter, if she starts wrong go back to trot, collect and ask again until she gets it.
make sure she is bending to the inside and give the cue very clearly!
Ive had the same problem!! Make sure you are cueing her before you ask her to pick up that lead, by tightening the rain and moving her nose slightly to the right. Also LOTS of practice helps ans she will eventually get it over time!!
Really try to squeeze with your leg and see if that works