That is a very involved question

. I noticed this as well in this video. I am not an expert, but this dog appears to have a jumping issue. It could be something that was poorly trained (I know of a dog that was clicker trained to jump and has developed hesitations like this), it could be structural, it could be genetic, or it could be a vision problem.
A lot of times this is referred to as ETS, or Early Takeoff Syndrome. I am again, no expert, so I can't say which of these is the case for the dog in question. If you do a google search you can read Linda Mecklenburg's study on ETS as well as see video examples. I believe there have also been a few topics on this forum.
So to wrap this up, the answer to how you fix this depends on what is causing it. Could be as simple as jumping drills (grids, etc) or as complex as dog contacts for their eyes.