Wait for little while and try again tomorrow. Sometimes you just 'forget' how to do something. You don't want to keep doing it wrong and instill it into your muscle memory.
Could you ever do it before? Or are you learning the moves for the first time?
Because one day I kept going over to the side for no reason and I couldn't do it for some reason when I frequently did it right before. THe next did it was fine though.
If you're learning it for the first time, try it one an armrest to a couch or something and just keep your hands on it and pull your legs through. See what's wrong. Try it on a couch first so if you faceplant its not a big deal. It's probably just fear. If not, it could be that you're not tucking enough, or your arms aren't giving enough lift. Practice whichever one is your problem.
