I've known a lot of trainers that tell you to counterbend to pick up the correct lead, and yes it does work, but its very bad because then you have to counterbend them to get them to do it and in a show that can really cost you. My trainer always has us pick up the inside rein or the rein I'm turning with at the same time I cue the canter, that way they associate that direction and bend to that lead. If he won't do it on a straight away it's because he doesn't have to. On a turn, they need the balance, but on a straight away he doesnt. Maybe if you work on the cue over and over again on turns or in a round pen, he'll pick it up better. Or work on transitions. Correct if he gets it wrong and reward when he gets it right (usually best reward is stopping). Hope this helps!
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