I can laser, inward heel, and varial heel, but I can't heelflip.
I know it sounds weird, but I pop, then I slide up and out away from me, and it underflips.
Help with flicking?

Off-topic: 100th post!

make sure you have your fingers curled, a lot.
On the edge of the board closest to you, although you might want to experiment with what front finger placement works best for you, I find around the front bolts is best.
Something to remember thats different from all tricks, but that you don't need to remember with lazers, inward heels, and varial heels, as I also find all of those tricks easier:
With a kickflip, or any flip, as you pop your hand sort of goes towards your chest. Like, away from the board a bit to let it flip. With heelflips, make sure you DON'T do that. Keep your hand ABOVE the board, and as you start to flip your hand/arm is like a tunnel over the board.
Give it time to pop, the flip out and DOWN as hard as you can. Especially at first, you're gonna have to try REALLY hard to flip it, it'll get easier though.
Then just try to land it, and roll away

I hope thats some help. I find them hard too, nollie heels and sw heels are way easier imo! I wish I could get my heelflips small and controlled, but, they're a weird trick in fingerboarding.
Hopefully with this advice you'll be able to land them at least, then work from there