I'm having issues with the ball of the foot roundhouse kick. The instep kick is not a problem for me, nor is the setup. My issue is with getting the toes pulled back far enough so that I can hit with the ball of the foot. The kick isn't new to me. I've had both roundhouse kicks in my arsenal for several years, but we've never really spent much time on the ball of the foot roundhouse kick in class. This issue came up when I tried three times to break a single board and nearly broke my toes instead. Since then, one of my instructors has been working with me on the kick and he told me that part of the problem is the flexibility in my foot. He checked to see how far I could pull my toes back and told me to pull them back more. I couldn't. He pulled my toes back for me to where they needed to be, but I can't keep them there. He told me I have the stiffest toes he's ever seen. Trying to get better at this kick is driving me crazy and frustrating me to no end. Every time I practice this kick on my own on the heavy bag, I seem to hit with the toes. I need advise as to what I can do to get better form and flexibility.