Odds are high that whatever the problem is, it's occurring before the gymnast leaves the springboard. Here are some things to look for:
-Run speed. Most of the run doesn't matter that much, but you want to look at the least three or so steps; those should be top speed.
-Hurdle distance. The hurdle should be fairly long, and the gymnast should contact the board just behind the line. You'll probably have to fiddle with the gymnast's starting point a bit to get this consistent.
-The gymnast should have her weight behind her feet when she contacts the board.
It could also have to do with board settings.
Are there enough springs? If the gymnast is bottoming out the board, throw in another spring.
Is the board close enough to the table? There's a common misconception that further = better, but in fact the opposite is true; the closer you can get the board to the table without compromising preflight position, the more power the gymnast will be able to get.
Hope this helps!