Does she know how to canter on the lunge? Perhaps you could have someone lunge you both and then ask for it...

I teach mine by continuously giving the cue and trotting them until they pick up the canter. Even if mine do it erratically, for just a few strides, or with hollowing out, I don't mind as those things can all be fixed as training progresses.

I have to note too though that I do teach my horses to ground drive WTC before I get on so they know how to move correctly, stop and turn way before I get on. I think it makes all the under saddle work so much easier for everyone.