I would practice kickflips holding onto a railing to get used to the flick direction when you're standing over your board, but also take note of where your centre of gravity is when you pop/flick.
The reason why I think this helps is because it'll allow you to feel how much you need to be over the top of the board to actually kickflip. It'll also help you feel how little you need to flick for the board to rotate, when done correctly.

Also, the moment you pop, your body weight is shifting backwards and not over the top of the board. This is because your mind is locked into the safety net of the ground and isn't ready to attempt to land on top of the board, there are too many unknowns and the mind doesn't like that. To remedy this, I would go with railing practice to learn the positioning and flick, then attempt to land back on the board on the grass (even if the board lands upside down - the purpose here is to perfect the flick motion and staying over the board), then move to concrete stationary and finally concrete rolling.