Boy am I pulling my hair out trying to figure this one out. My TB normally loves to canter. He's like the energizer bunny. He'll go all day. As of 2 weeks ago that changed. For reasons I cannot diagnose, he does not want to canter, either way. I do not think it is due to any type of soreness. He is not showing any obvious signs of resistance. No ear pinning, tail swishing, head tossing, nothing. He will offer a fabulously forward collected trot in place of cantering. I doubt he'd put himself into a frame if his back were sore. It is not an issue of being unbalanced. He doesn't trot strung out and faster when I ask for canter. He just offers a bigger trot that would score a 9 from any dressage judge. It's like he's totally forgotten how to do it. The only thing that happened around that time was new shoes, but this farrier has never done a bad job, there's no heat in any hoof, and the horse is completely sound at the trot. He did recently go through an episode of sleep deprivation and was falling asleep on the cross ties, but that was quickly resolved by moving him to another paddock. He was also diagnosed with a heart valve condition, but the type he has is very common and is not likely to affect his performance. I've been checking his heart rate during and after exercise, and if anything, he's on the fit side. Any ideas on what this resistance could possibly be? I'm baffled.