I ride Huntseat, and how I ask my horse to canter is:
1) First of all, if he’s not paying attention, I give a half-halt.
2) I give him a little bit of inside leg at the girth.
3) I give him pressure from the outside leg behind the girth.
4) I make a kissing sound almost, only then should he canter.
5) If he doesn’t canter right away, you will get a tap from the crop.
So I have a couple recommendations and questions for you.
Earlier you mentioned that you used to ride Western. Did your horse pickup the correct lope lead in Western riding? If so, what were your cues? Did you change your cues from Western when you transitioned to English? If you did change your cues, try and ask your horse to canter with your Western cues. Or if you still have your Western tack, try and get him/her to canter in that. You could also try to canter your horse on the lunge line, if he/she will pick that up. People have also mentioned that there could be physical problems involved, so make sure that a) she/he is having any hoof problems, b) she isn’t in any pain from the tack (saddle not fitting correctly), and c) her back or legs just are feeling out of alignment.
Hopefully this helps:
Evention TV’s “Correct Canter Lead” video: https://www.youtube.com/watch?v=WPDPzJelRj4
Good luck and feel free to ask any questions!