It may be a combination of both. Letting off your edge pulls you more toward the boat on takeoff which can lead to you landing more towards the boat (on your heels) on the landing.
The biggest thing that helped me get these consistent was doing a very, very late handle pass. I pass the handle right as I'm about to land. Over-rotating is what usually causes people to slip out on their heels. If you throw these w2w and pass the handle late, then you'll have no problem. You need to get a little forward on your toes on the landing, but if you do the two things I mentioned then you really shouldn't have to worry too much about that.
So do the first 180 on the way up, pull the handle to your back hip on the way down while you spin the last blind 180, and then pass the handle as you're about to land.
Let us know how it goes! You don't have to spin nearly as far as most people think so do them slow and controlled.