Horse that ignores the canter aids

Hello. The first step, I think, would be to have the horse checked over by a vet to make sure everything is as it should be. Several of the things you mention, that she gets grumpy, will not listen and is very inconsistent in her behaviour, suggest that something may not be quite right. Beyond that, two things come to mind. One is whether your horse understands what you mean, and the other is that it is hard to be precise with your timing of pressure and release.

To make sure your horse understands what you mean, be careful that your aid goes through. Think of it like this: start with the smallest signal or pressure you want your horse to respond to, and take it away the moment your horse responds, which is often sooner than you would think. If the horse does not respond, increase the pressure until you get something close to the response you had in mind. Raise your expectations over time for how well the horse responds.

You can describe it as the 4 Rs:

Request, the signal or pressure you use every time you want the horse to do a particular thing, for example the leg to ask it to go forward.

Response, the horse responds as you intended.

Release, the easing off that teaches the horse to give the same response to your request.

Repetition, repeat until the horse understands, then take a break before you ask again, in a little while, later the same day, or in the next session.

Teaching a horse the canter aid, for example, is about building the training up in a way that makes sense to the horse and expecting a correct response 100% of the time you ask, over time.