My dd has this issue from time to time. She was with a college coach, but mainly for fielding and hitting one day, and the coach knew she pitches as well and wondered why she wasn't out there to warm up, my daughter told her that she had been hitting her hip, had a nice bruise on the inside of the arm, elbow area, and needed to rest it. College coach told her a drill that has worked, she told her to do it from time to time to force the hip to get out of the way of the arm, meaning that they're going at the same time. Tell your dd to stand on the rubber on one leg (she's right handed so she has her left knee up) and arms straight out. She then pushes off and throws the pitch. My dd is now doing this when she warms up for a game, just to be in "check". Hope this helps and I'm sure there's plenty of others that can give you a different drill -- looking forward to reading those as well.