I agree w/ you on lack of hip extension. It is hard to see from the video, but you look really quick when you lower it to mid-thigh and you may be bending your knees a bit instead of getting your hips back. This may be what is pushing some of your weight forward and leading to the early extension w/o getting a bunch of force into it.
I would try lowering it a little slower, making sure your hips are getting back and your weight is on your heels.
When I do them, I really feel my hamstrings and butt loaded up. I don't know if you feel that or not, but it may be something to look for.
Maybe try standing ~8" in front of wall w/ your back to it, then let your hands go down your thighs as you push your butt back to the wall. When you get to the loaded up position, wiggle your toes. It might help to reinforce the movement pattern.
Thats what I got, take it with a grain of salt.