What to do if your horse doesn’t listen

You've trained a behavior with clicker training and its all going well until... one day you cue and your horse doesn't do it.

First and foremost, check if they physically CAN do it? Sounds silly but i have seen my students cue our horses to back up without consideration of the wall behind them so look around, is something blocking them? If not externally, then internally? Is there a physical thing bothering them? If a horse backs up happily on cue with R+, then suddenly doesn't, or begin adding signs of discomfort like tail swishing or head tossing, it's very likely the behavior is causing them discomfort. It's best to get a vet or chiro out.

Can they emotionally do the behavior? Is there something upsetting about this behavior? Is something upsetting happening around them? Has conflict become a part of this behavior?

If there is a major distraction in the environment you need to work more on having reliable cues with distractions. This means putting more reinforcement history (be more generous) with the behavior in easier scenarios and practice it in steadily more distracting places. Also consider though if these distractions are for safety, if the horse is very afraid, this isn't a good learning situation, break it down into smaller steps.

Sometimes we don't realize that a punisher (an aversive) has crept into our training. For example, all my horses have a reliable "back up" on visual and verbal cues. The only time many of them struggle with this behavior is when being asked to back up from their stall door. Why? Because it results in their door being shut, which they find punishing. So my reinforcement needs to outweigh the punisher in this one scenario. I may not need to heavily reinforce back up in most situations now that it's well established, but in this situation i will always reinforce it heavily, to help contradict the punisher that i can't prevent.

Conflict and coersion can creep into our training in lots of ways. If we cue a behavior and ask in a way that is aversive/uncomfortable to the horse, where they feel forced or coersed, it can lead to behavioral conflict. We'll see tell tale signs of conflict. So pay attention!