The problem you have is that she is anticipating the jump. This is more common when you are against the clock and go slightly too fast for the horse. You could go two ways: Some people might put a stronger bit in and become more assertive; I would recommend you slow everything right down and go back to basics. Complete some rounds trotting until she can calmly complete several rounds. You can also give her more to think about, such as through pole work. In the long term, you might think about having her back teeth checked to make sure there is no discomfort.