As a past riding instructor, I think you need to back up a bit in her training. If you are using a really strong leg on her, and she is not responding, then there are a few things you can try. I would also start carrying a jumping crop (a very short bat or quirt). You don't actually have to use it on her, but if she knows you have it, she may be more consistent.
Try lunging her over small jumps first. See if she hesitates when there is no rider. It might be that she has gotten chucked in the teeth with the bit in the past after a jump, and she is still anticipating it. If she is fine over jumps lunging, then progress to the next step...
Next I would try trotting some small jumps. The trot is a much different gait than the canter, and it is much harder for a horse to really slow down and lose impulsion at the trot. Don't try to trot much over 2 feet though... also place some trot poles in front of the jump to keep her at a trot. Try to land in a canter though...
Also, in addition to the person above who suggested cavaletti, I would also try jumping her over gymnastics... these are jumps set up with no stride in between. Sometimes they are called "bounces." They need to be carefully measured out though to suit the horse's stride or you can get into some serious tangles. Make sure you put a ground pole in front of the first one. Gymnastics MUST be jumped at a canter...
Hope some of these ideas help you out!
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And PLEASE, please do not follow DoveRydur's advice... you should never, never, ever jump alone, especially on a horse who is already having issues with jumping!