The flight in the long jump is somewhat difficult due to the need of the athlete jumping to properly perform multiple parts of the takeoff correctly (I recommend watching the video link above before moving on).
In the takeoff an athlete must keep their eyes and chest up when jumping from the board (which often does not happen). If the athlete fails to do this and their eyes, chest, and overall posture are trending downward they will have an exceedingly difficult time controlling their flight. When this happens athletes generally put their legs/feet down quicker than desired and thus cut off possible added distance.
Many coaches say that once an athlete has jumped there is no way to add any distance to a jump - everything has already been determined due to the approach and takeoff. This is true, but many times athletes can do improper technical aspects (in flight) that greatly reduce the chances of them maximizing their best jump and losing a great deal of distance.