My horse recently went through a week long phase where he changed from being better on his right lead canter, to being unable to pick it up at all.
I did a lot of exercises on a 20m circle, making sure he was set up VERY well before asking for the canter- correct bending, not hollow, listening to the aids, etc. and only when he was all there ask for the canter. I have a tendency to be a little weak with my outside rein, so I also concentrated on making sure I had a solid connection there. And for the times that he still picked up the wrong lead, I'd immediately go back into a trot and then immediately ask him back up into the canter.
He's over it now, but strangely he now seems to do better on his left lead (easier time picking it up, maintaining the canter, better balanced, etc.). I can only assume that it has to do with the way he's muscling up, as he was a little underweight and very undermuscled from having the winter off when I bought him. His right lead isn't bad, it's just not as nice as his left