The canter is a three beat gait. Many, many Walkers are bred to pace which is a two beat gait (because the pace is easy to "square up" with devices). So what you're got is a breed with lots and lots of "two beating" horses that often have trouble learning to three beat. There are many ways to deal with this, but it takes knowledge and skill to do it effectively.
If the horse has this issue it's the people who bred these horses that bear the responsibility. Breeders who concentrate on a true running walk (a four beat gait; there are a few breeders who do this) have versitile horses. The transition to the canter for these horses is a non-issue.
So if you want a Walker that does more than gait (badly) then you'll have to look long and hard.