It’s possible to jump too high on a heelflip. They require a more restrained upper body movement than ollies or kickflips, I remember correctly.
I would also focus more on kicking off the toe side than towards the nose. Your first heelflips probably won’t be very high and they probably won’t be caught. You can focus on kicking towards the nose and lifting your knees more when going for height, after mastering the flip while keeping the board beneath you center.