Yeah it sucks SG! its not like I roll on to my tip-toes, just as I go lower, my weight transfers forwards to the front of my foot gradually and eventually my heel will come off the floor slightly. Its bizarre and only happens once I put some real weight in to my squat. All the same, the biomechanics guy I've had check out my squat said its something I'll have to overcome, but certainly thinks it is doable.