I had the same problem with a little pony we have, I've found that extending canter down the long side helped. I would also suggest getting physio/sports massage to release the muscles
One more suggestion is poles on long distances so she has to use her body to make the distance x
Has it been going on long? If it's been a while I'd first suggest a lameness test by a vet. There could be something going on physically.
Otherwise, I have a draft cross mare who is heavy on the forehand. Our warm up consists of a lot of transitions. Count 9 steps, trot, 9 steps, walk. I usually repeat it a handful of times. Make sure she's listening then start decreasing the steps between transitions. Then I move up to trot - canter transitions. It will encourage them to use hind end and should help her grow the muscles in her hind end.
All the best.