Lunge her over small jumps (which don't have wings), introduce another jump to make a small oxer. Make this no more than 1.5 feet high but make the back jump a bit higher than the front. That will encourage her to spread out better. My horse gets a bit lazy when jumping, doing only the minimum and so I occasionally do this to get him thinking a bit more about the jump. As she gets more confident, make thema bit higher. When you feel she has got the idea, ride her over the smaller jump and don't interfere with her mouth as this will make her jump short.

Hope this helps