Just an add on to what was already said -- if you are having stopping issues, it's even more important to sit back and wait for the jump, and the give a big release when he does jump.
If your horse isn't 100% confident, he's going to use your poisition as an excuse to stop. I have a horse that will stop dead if you get slightly ahead of him or if you don't release. Which makes sense, because your weight is now over the whithers, and bearing down on the front end, which is heavy enough to rock back and get into the air, but when you add your weight plus the fact that he doesn't have freedom of his head, he's gonna stop.
Not so much criticizing, just explaining. :)
"The white horse moved like a dancer, which is not surprising: a horse is a beautiful animal, but it is perhaps most remarkable because it moves as if it always hears music."