The Direction of Jump
Mechanism / Why It Happens
If your body leans too much, you'll miss the board. Keep your weight shift to a minimum.
In a Varial Heelflip, the board tends to move toward the heels, much like a Frontside Pop Shove-it. However, if you lean too far backward, the board will instead shoot toward the toes. As explained in this Tip (Link), it also becomes much harder to make the board flip. Avoid jumping too far backward, and keep your weight shift to a minimum.
What to Do
Trying to learn the weight shift and the flip at the same time makes the trick harder. Practice them separately at first.
If you can't stop yourself from leaning backward, try doing Ollies with your Varial Heelflip stance.