We've been talking a lot about this in the Weight and Resistance training forum.
The prone jackknife is something I had a major issue the first time I tried, but I'm getting a bit better.
One thing I found is you may need to build up with it. Lie on the ball on your stomach, and roll out as far as you can hold yourself up (your arms and belly may quiver). Then pull the ball in with your legs. If this means, the ball is at your thighs then so be it. Each time, try to roll out a bit further to perform the move.