Some horses just don't know how to canter away from a fence too small. I knew a horse that was so well trained in western pleasure that when we turned her english, no matter how high the jump was, she wouldn't canter after the fence unless we kissed in the air.
Make sure you don't get her completely out of trotting after the fence. Mix it up. I've known plenty of horses that learn to canter after a fence and they eventually start to canter before the fence or take off after. You want her to wait to the base and land comfortably.