The Heel Extension
Later, I also found that the reason he couldn't continue with a correct hip-flip after the step-flip was toe lift. In other words he fell off from the step and didn't lean back well, but fell straight backwards. It was caused by lifting the toes, that is tilting the whole legs too much backwards. Ideally, this could probably be avoided by placing the spacers correctly, as I said above, but I found a much easier way to correct the problem and it worked well. I just put a bit of extension on the heels. If I make another acrobat, I will make his heels longer.
Extension of the heels to prevent toe-lift.