Pull your feet up higher and slightly jump backwards

try and jump far behind it and catch the board in front of u then do it two feet :)

When you do the varial heelflip if you have the flick then you should do it just like your front shuv when you pop it push in front of you

I have my own foot position for it. I use my nose so my board doesn't land backwards first off. I like to point my flick foot towards my pop foot because I believe the flip comes easier. For my back foot, I put it in the pocket of the tail and use my toes only to pop. Just focus on the front shuv and if you flick it should form. Just pop it forward with your momentum.

Jump back when you do the trick and keep your body more straightened out dont keep yourself hunched

Sometimes I have this issue as well and most of these replies have the right idea, it helps to really focus on the jumping aspect of the trick especially if the flip and everything else is fine. Really focus on bringing your legs up and even tucking them into your chest too. If the board keeps going behind you, make sure you have your center before popping. You don't want the board leaning too far toeside or heelside. When you rear down to pop think of it like sitting in a chair. It really helps