As for your canter it could be a balance issue. Another common mistake I see is the horse having the energy, but the rider not able to contain it. Don't let him rush forward, hold him and bottle that energy up into the canter. Also, make sure your not doing anything that would hinder him, for example posting, leaning forward, etc. Make sure your cues are solid and consistent. Personally, I live verbals (noises) for transitions to make it all the more clear. I finds it helps a lot when training or tuning a horse.
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