This is very common. Often because at full motion she is throwing to you on a bucket. Correct? Hang a tarp and get her to throw full out into a tarp a few feet away. This will focus her on brush, posture, and all things mechanical and not on the results of the pitch. Throw only into a tarp every day for at least a week. Keep at it until brush is always there. Then have her go full to someone standing at full distance. If she can maintain brush then and only then should you squat. If she regresses go back to the tarp and start the process over.
I was debating setting up one of our hitting nets for the pitcher I'm working with, but I wasn't sure if this would be a good idea or not. Sounds like it could help her focus on what we're working on and not the end result of the pitch (I stress process over result to my players, but in a shocking twist, teenage girls don't always buy into that concept).