Next time you jump, try to feel if you are gripping with your knees. If so, you need to work on gripping with your calves. Try to hold a 2 point position without your legs slipping back. Once you can do this once around the arena, try twice, then three, then four, then more. Then you can start practicing again over small jumps. If you feel yourself pinching with your knees, go back to 2 point until you feel ready to try again.