One other thing you may need to do is have the pitcher throw into a screen or net rather than to a catcher. The reason is psychological.
One of the most common problems I see when trying to teach fastpitch pitchers to learn to whip the arm through the release zone is overcoming the urge to aim.
They’ll be doing a good job of bending at the elbow and letting the upper arm lead through the back of the circle (rather than pushing the ball down with their hands). But then, right before they get to that critical moment where the upper arm slows down naturally and the lower arm passes it to create the whipping action, they will instead let the ball get ahead too early and defeat the whip.