Training Science Articles:
One of the most valuable aspects of positive reinforcement training is our horse's ability to participate freely. They have the option to partake in the fun or they could say "no" or "i'm not ready for that yet". Better yet we have the ability to listen without risking reinforcing the wrong behavior by releasing the aversive if they say no.
But what do we do if they say "no"? We still want the behavior, the horse still wants to perform the behavior, but for whatever reason they chose not to. We must first find out what's causing them to say no.
First we need to check all medical reasons. I recently found out the reason my colt was having SO much trouble with hoof work was because he had neurological issues. Be thorough, ask a vet or specialist depending on the situation.
Check the environment. Is the living situation ideal for that individual horse? Appropriate turn out time? Appropriate company? Healthy amount of stimulation/enrichment? I'd say a huge amount of the time if a horse says "no" without a physical reason it's because there's something wrong with their environment.
Last but not least you want to check the training itself. Is your timing accurate? Treat delivery clear? Rate of reinforcement matching this stage of the skill? Value of the reward matching the complexity of the skill? Does the horse realllly understand what you want? Are they being overfaced (the skill is too difficult or scary to do)?
Remember training isn't just about the click and treat. Keep your horse's whole life in balance and the rest should sort itself out