Hmmm... let's see if I can remember back 5 years
When my guy was 4 and learning leads I always set him up in a shoulder-out before asking for the canter. He wasn't balanced enough to keep himself bent the correct way yet. So, shoulder-out and then ask for the canter. Most of the time he got the lead, since his head/neck was counter balanced to the outside (if that makes sense
). He got lots of pats and "good boys" for that. But, if he didn't then I just brought him back to the trot right away, set him up and asked again.
My horse has always had minor balance issues at the canter, we're actually just now resolving them... so yours might not need the whole shoulder-out thing, but it's worth a try if all else fails.