Oh cool, I'm glad you're taking him to the chiro. They can find things that vets can't!
Anyways, do you ask for them going straight, or do you ask for them coming to the center of a figure 8 or serpentine? I always teach flying changes on a figure 8, much easier to coax them into it when going into a turn than on the straight away. I know more people that train them on the curve though.
Another way is to pick up a canter, and once you come around the oval, canter straight across to the corner opposite you are facing, on a diagonal line. As you head into the corner and just start to turn, cue for the lead change. They pretty much HAVE to change, or run into the wall.
I know you said you've had him for awhile, but maybe you need to go back to simple changes to reinforce the basics? Canter the figure 8, come down to a few strides of trot, and then ask him to move off. Once he's confident in that, make it one stride less before you ask for the other lead, and so on. Get it until he's comfortable coming to the trot, trotting one stride, and picking up the other lead. Then, ask for the full change.
If anything, it'll keep his mind working and his feet moving, maybe he's just anxious about doing the change or not strong enough to do it yet. Maybe start every session with the simple changes so he doesn't anticipate it, and throw in the full lead change when he's not expecting it every time.
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