I used to have this problem but i sorted it out.
I would always land ahead of the board, slightly at the side or sometimes i'd land back foot on and front foot way ahead of me off the board.
The way i do them now is to pop as hard as you can!
I always pop AMAZINGLY hard or else i won't land it, it's somthing about being above the board when it low off the ground, i hate the feeling so i always make sure i'm really high as i come in to catch them.
If i don't don't pop above the height of about 2 or more decks then i can't do em.
The other thing is, when you pop, lean back the tinniest bit.
And make sure that when you go to pop you're not leaning forward to much.
As you pop lean back slightly to let the board flip under you then catch it.
My heelflip method is a good way (also consistent) because my Heelflips are the same height if not higher, and as clean as two guys who have been skating 6 years and a guy who's being skating 7-8 years.
(I'm not trying to boast but that's just the way i do my Heelflips)
I hope this helped you and i really hope you have Heelflips dialled soon!!
Just use my methods and PRACTISE!! honestly thats the only way if you go out and try nothing but Heelflips for about an hour, using my methods you should get them.
Good luck