I went through this one with Cougar. He'd throw the RANKEST bucks known to man kind. Heels over head, I'm going to launch you off, types of bucks. Was not fun. At all.
Anyways, what are YOU doing when you ask the horse to canter? I find the best thing for those youngsters that chuck in a buck is to push them forward. They're all stuck up in a little ball of tension and instead of going forward they kick up their toes.
If your trot quality isn't there I'd say forget the canter for awhile. She needs to feel comfortable at the the trot before she can canter. I'm not saying baby girl needs to have it all together like an Olympic level dressage horse, but she should be nice and relaxed at the trot. Moving underneath herself and accepting contact. Otherwise she's just riding blindly into a canter.
That's my two cents. I'm sure other's have advice too that may make more sense than mine.