If the Timing Is Too Early, It Won’t Flip
Mechanism / Why It Happens
The board becomes easiest to flip when its upward force weakens
When you pop the tail, the nose rises and pushes back against your front foot. You may feel like flicking right after the nose starts rising, but immediately after the pop, this push-back force is very strong, causing your ankle to lose against the force and preventing a proper flick.

What to Do
Pull your front foot in sufficiently before flicking
After popping the tail, let the nose push your front foot back and pull your front foot in so
Analyze
Check your front knee angle right before the flick
To confirm that you are pulling your front foot in enough before flicking, check whether your front knee is sufficiently bent right before the flick. From this position, extending your lower leg around the knee will allow you to flip the board effectively.