I agree with the Kay, she has got the muscle there, just doesn't want to put out. Horses often come into a speedy, unbalanced, poppy trot when they don't want to canter, for whatever reason. Make sure your lunging is correct - you are following her hip to drive and come behind that with a half lowered whip, and you do not run in circles, you stand your ground and pivot, only yeilding to the occasional loss of bend. Your shoulders are upright, your attention is fully pointed at her hip while still looking at the big picture to evaluate balance and soundness. If you are lunging in round pen, you need to take a few steps up toward hip as she trots (making her feel more 'pinned') ask for canter/lope, and follow up with whip. When she canters, praise her while not letting her stop immediately. She needs to get a few strides out! If you are lunging in pasture or large ring, come up against the fence when you ask for canter as it will encourage her to feel she doesnt have a choice but to move up. When she gets out of sorts trotting to fast for too long, DISCOURAGE IT and make her come to a collected (or semi-collected, since she's out of shape) trot and try again. The first time you can race her into a canter but after that you don't want to encourage that choppy quick trot before a canter.