Sounds like your horse is doing exactly what mine is doing!
When I first got him, he had a great canter, but he is out of practice, so I have been working on improving it for the past 4 months. One thing that has helped immensely is to get him in shape; great shape. (He got slow and fatter over Winter!) This of course helped ALL his gaits, but it is imperative in the canter.
I ALWAYS "kiss" when I want a canter. Of course you can make any noise you like, but by being consistent in this, he knows what I am asking for, and I don't need to spur or use a crop or anything else. He knows what I am asking for when he hears that sound.
Then, he started cross-firing. So I worked on a lounge line, and in the round pen. I let him go faster when I am not on him, just so he can find his comfortable foot fall. Then, he slows down naturally (tiring) and usually at this point, he "finds it". Then he gets big praises and a break.
Also, on the trail, most horses like to canter up hill. So I would go out, "kiss" and let him canter up a hill, and again, give him praises. Not only has this helped his rhythm, but again, it has really helped him get in shape.
FINALLY, yesterday after 4 months of work, he cantered on the correct lead almost all the way around the arena, slow and collected. It has taken a lot of patience. But my guy is willing, and he is a horse who tries very hard, so being patient is my pleasure.
Hope this helps.