Yep; this seems to be an ongoing problem with some gaited horses. Some do their lovely "rocking horse" canter, and then others... just have difficulty.
My mountain horse can canter fairly well with his left lead, but on the right lead, he sort of does a "skipping" canter. The trainer I have been working with seems to think he has "too many choices in his rolodex"
; he can walk, trot, running walk, pace, rack... And even when he does canter, many times it has an extra foot-fall in there, or he holds his back leg up for additional time. When he does get it right, keeping him in it is also a challenge.
We have had more success getting him to canter from a walk, and NOT a gait. Or, when he trots, if I start to post, that seems to get him cantering as well. Right now he is having more difficulty, coming out of Winter when I have not been able to ride him as much (too much snow here). Being in good shape makes a world
of difference. Lounging has also been really beneficial in keeping him in it, and helping him know what is expected.
Also, watch him when he is out in the field. My horse does a perfect canter when he is out running and cavorting in the field, so I know he has it in him. Just keep working on it; what works for some doesn't work for others. I will be watching this post to see what works best for you!