If your horse does not pick up the right lead, it isn't always due to poor training. The right start for a correct lead begins each stride with the outside hind leg, ending with the inside foreleg. At the riders cue a horse is encouraged into sequence, and should the lead be wrong, the rider will quickly recognize it. A quick return to the trot usually fixes the problem by asking for the canter again.
It can be frustrating when the horse regularly picks up the wrong lead. While there are many techniques and tricks to get the horse to respond and perform a specific lead, one thing must not be ignored: Your horse may have a definite physiological reason for consistently picking up the wrong lead.
Naturally, a horse prefers to canter on the correct lead because they feel more secure and balanced. At the canter or lope, the trailing foreleg resists the centrifugal force that pulls the horse to the outside of a turn. When horses play in the pasture they usually choose the right lead when cantering turns, they may even switch to it to bring stability. Even young horses will most often pick the correct lead automatically.
So, when a horse resists cantering on a specific lead it is usually caused by physical pain or difficulty. This could be due to stiffness or discomfort of the back, body or legs. It may not be obvious at a trot or walk that the horse is overtly lame, and they may even be able to pick up the lead with a rider. But, if they consistently are not able to continue it could signal a physiological problem, or maybe even a malfunction in the horses neurological system.
If you are noticing these issue with your horse, stop
re-training and make sure through a veterinarian and farrier, that the horse is
free from these concerns. It may take several months to diagnose and
correct lameness or neurological problems, but it is the only way to help your
horse get back on track.