How can I train a canter transition?

I have started to share a 13y OTTB who ignores leg aids for canter and will not transition in the school without a messy mix of dropping her shoulder, sideways hops, head back and turning in. She is ok on the trail and canters at any opportunity, usually from a straight line. She is trained for voice command but I suspect not ridden much as she is stiff to get round the leg, difficult to profile and everyone is frightened of her in the yard where she has been for 5 yr. She is 17.1h and opinionated. In the school, once you ask for canter, from then on she will stick her bum inside and trot sideways in anticipation to spoil your next request. Esspecially on the right rein. I have started a training program which recommended getting her to turn her head in without moving forward, which is OK, and separately tapping behind the girth to get her to yield her hind leg underneath without moving forward. She is totally dead to this and to any combination of taps, pressure and kicks.