I've learned that if your horse doesn't pick up their feet, to wear open front boots..if you wear boots at all. That way when the horse does knock the jump they can feel it and be like, "Woah I don't like that so I'm gonna pick my feet up next time!"
Also if you put a saddle pad on the pole the horse will see it and pick up their feet even more. I haven't tried this yet but I've been meaning to, I saw a youtube video about it once and the pony the girl was using was definitely tucking their feet from the beginning of the video where the horse was just knocking it off when there was no saddle pad.
I've also heard that gymnastics will work just like FSHjumper said. Engaging your horse in a lot of bounces, one strides, two strides, and changing things up will make him want to focus more and pick up their feet, and not be bored like a lot of people said.
And last but not least, have you tried adding ground poles? When there isn't a ground pole my horse usually knocks the jump. Also try putting a pole about one stride out, so he has to go over the pole, then jump. It will almost be like a bounce, but with just a pole, then a jump. Here are some videos of my horse doing the pole thing.. We actually have the pole on the landing side, when I'm telling you to put it on the other side.
Have the pole about a stride away so he bounces into it, then the jump.. Good luck