Just thought I'd toss my 2 cents in.
I have a "lazy" horse that was hard to motivate to canter . Usually once he'd made that transition a couple of times, he'd warm up to it, or the idea of it. Anyhoo, when I was training him (maybe it's my training that made him lazy, too :roll: ) he was slow to learn his leads and to figure out how to not cross fire. He was what I call a late bloomer (so much for being a descendant of race horses

). It took a lot of conditioning to get him to figure out how to pick up a canter. I don't know if I would make a huge deal out of it just yet, just keep building those muscles and don't quit trying until he gives you a few strides at least.
I found that slapping with the reins on the rear did more to motivate that anything, much like you would use a bat. I got careless with my spurs once out of frustration and he bucked, reminding me that wasn't very nice.

But I always used my regular cue before resorting to the rein slap (and I have really long reins, so I knew I wasn't pulling his mouth at the same time) and he got it after a while.
Also, I find if I pushed him into a fast trot and did't LET him canter, he suddenly found that trotting is a lot harder than cantering and willingly wanted to breakover into a canter . A little reverse physcology.
My lazy horse still requires more effort to get into that first canter of the day than some horses, but we're talking just a firmer leg cue and a split second slower to pick it up, not a significant delay. He just makes SURE I want to run first. :roll: