I have to go with pikes on this one too. I can start to get my butt in the air but a soon as my upper body starts to approach being perpendicular to the floor, I tumble right off the ball. I don't know if I lack the shoulder strength or if my feet aren't positioned correctly on the ball, but something isn't working.
I'd also like to get more than a 6 inch clearance from the floor on tuck jumps.