Hello, Wow I know how frustrating that issue can be! There are a couple things I would suggest for you guys to do in order to get to the root of the issue! First off you need to make sure that it is not a problem with pain..Have your horse and saddle checked by a chiropractor/acupuncturist, it can be the smallest things that can cause the biggest problems.
Now here is a big question does this problem with leads only occur in the arena or is it as well out on the trails? Then from there depending on what the answer is and what was said with the chiropractor, I would start out with simple stretching and bending exercises, Carrot stretches are great on and off the horse, it’s important though to allow the horse to stretch not you stretch for him. He has to go as far as he can go and hold it there as long as he can. Then going into the arena with the mind set on what you want to accomplish is a plus. If it is not medical then I might suggest that even if the canter is only two strides on the correct lead stop and call it a day with tons of praise and cookies gradually lengthen what you are asking for. Circling and figure eights are wonderful to do to encourage balance and lead changes. I would also put a small cavaletti pole out and change direction over it preparing to switch a stride before over it. Try those things out and I hope guys can fly on the right lead soon!!!