I'm aware that I am probably going to get yelled at for this. But that's fine.
This is what I do with my horse. Wyatt can be pretty lazy but he has his reasons- a fellow rode him extremely hard when he was a 2 year old, we're talking miles of nonstop fast canter, every day, just rode the snot out of this horse. Luckily, it didn't injure him, only his ambition, which we are slowly working up.
But I can't let him get away with just not listening to me. He has to listen to me- I live on a ranch, and working cattle, if one bursts out of the herd, and I cue him, and he doesn't move, I get yelled at for letting the cow out. He needs to move when I ask him to.
So when I cue him to canter from a trot, I give him a few seconds to respond. If he doesn't, I cue him harder (leg cues, leaning forward slightly). Still no response, only an agitated swish of his tail? Then I take my rein and slap him across the butt. (This is where I expect to get yelled at.) It does not hurt him, I only slap him hard enough for it to hardly sting. Just surprises him a little bit and then he breaks into canter. I work up the levels of aids- first I ask him nicely. Then a little firmer. And finally, it's move or else! (lol) Good ranch hands have horses that move really quick on light cues, because in order to get in front of a cow, or to sort or cut, you need a horse that you can maneuver in every way, form, or fashion. And quickly.
Cue your horse- then reinforce that cue. Pretty soon, she'll figure out that if she responds to that cue, she doesn't get a little slap across her butt, and she'll respond nicely to your cues. :)