I have tried most everything and what is working for me is to break some of the mechanics on a good IR fast ball. Grip ball tightly deep in palm, let humerus pass by mid-line moving release point forward, finish across body with pink up and palm toward catcher.
We started with a new pitching instructor about 3 months ago and she had DD #2 make a very similar change to her change up and it made a huge difference. She has always thrown a horseshoe change but couldn't get it slow enough most of the time. All she did was change the release point and the trajectory. It was her go to pitch for Fall Ball.
We have always thrown a 4 seam FB/DB, but my DD#2 is starting to play around with a 2 seam FB/DB and trying to turn it into somewhat of a drop curve.