How quickly are you swinging the yo-yo? It needs to have plenty of momentum to get entirely to the apex of the swing while the string is between your legs.
Can you do a trapeze over your right thigh? The yo-yo needs about twice the momentum compared to that. Well, at least how I do it.
Are you afraid of hitting your… “unmentionables”? If so, practice with a string with a bouncy-ball/Duncan foam ball. It won’t hurt as bad, and the physics are the same even though the weight is different.
Lean your upper body backwards. If you are doing the trick how I think you are from your descriptions of what is going on. I think that might help.
*So I just stood up and made sure what I wrote made sense, because I was only thinking about the trick.
The “trick” to doing the trick is that you have to pull the string right after it starts to swing up, exactly like you are trying to intentionally hit your previously-mentioned “unmentionables”. Then switch your feet. You don’t even need that much momentum to start with, you just have to apply upward force to the yo-yo through shortening the string that is between your legs. A bit scary, and pretty counter-intuitive.
Side note: one can also do this trick as a repeater while walking without ever touching the string with your non-throwhand. Instead of hitting a trapeze, just send the yo-yo for another around-the-world, and behind your right leg again. But the yo-yo has to be going in circles at a very uncomfortably fast pace for people that just learned the trick… Work up to that point!