I'm having a lot of problems with my sidekick. When I do it normally, I'm always hitting with the ball of my foot. When I'm near a wall and chamber back a bit more, however, I can hit with my heel, but it takes a long time to do so. I can do every other kick quite well, but the sidekick just eludes me. Is there some stretch or exercise I can do that will push that heel out, or is there some other solution to this? I'm already quite flexible, I'm 5'7" and can get my heel above my head in a front kick, but I'm lacking form for the sidekick and need some strength for jumps.