I've had a really hard time getting into the S mount. Whenever I manage to get there, I know plenty of chokes and can hit armbars regularly, just getting there is the problem. The number one thing that seems to happen is I drive my left knee up, and as I do that, they come back down onto their back more. This leaves them completely flat, and me sliding back down to normal mount. What am I doing wrong when I try and set this up?