I would have her checked out by either a vet or a chiro. She may be having some discomfort issues while engaging into the canter. If all checks out well by vet or chiro, then I would suggest putting her in either a running martingale or standing martingale. You can then collect the reins, get her going into a nice frame, then ask for the canter and keep her head down by the collection of your reins. It really doesn't sound like a training issue, but more of a pain issue.