Damn, I hate this trick. It's so inconsistent. So here's a small biomechanics tip that might help.
Firstly, the groundrules: Keep your arm straight at the start, and make sure your first movement when attempting the trick is
downwards - no sneaky tiny tug upwards first!
Right. So now a very small thing that has helped me a little: Think about the tension in the string. Obviously, it's really tight when the ball is hanging there, waiting for you to start the trick. Now, as soon as you start to move your hand downwards
[1], the tension in the string is released. At that exact
moment that the tension has been released, bend your knees as quickly as you can. The downward knee-bending will increase the downwards speed of your hand
[2]. Only a fractional amount, but it could be the tiny difference that helps you to land the trick. If you start bending your knees
at the same time as you pull your hand downwards
[3], you are actually moving the ball closer to the floor a small amount, making the trick harder! Perhaps that's really obvious to some of you, but sometimes the most obvious things are only obvious
after you have started to think about them!

I hope that helps.
Good luck!