I was explaining to a client yesterday why it's so important to differentiate between a sensitive but shut down horse, and a dull horse.
A dull horse is a horse who has been poorly taught to respond to aids or cues. He has simply learned that the stronger cue is the only real cue, and all the others are just the human confusingly flailing around. These horses have to be carefully re-sensitized to lighter cues, because they've learned that those cues mean nothing.
A sensitive but shut down horse is another matter entirely. This horse is constantly anxious or worried about what the rider on his back may do, so as a defense mechanism he goes into "auto pilot." He knows he isn't supposed to buck or run off, so often this means charging at a trot along a trail until the very end, or only really turning one direction. For some horses, this can even mean just shutting down entirely and stopping. The LAST thing you want to do with this horse is try to sensitize him - trust me, he is VERY aware of your cues. He just feels so incredibly overwhelmed by them he's just panicking and doing whatever he thinks is best.
With these horses, it's so important that you take your time. For example, the horse I was riding had a "woah" problem, couldn't back up, and couldn't bend left. Any time you asked, he would brace, lock up, and just stand in a panicked little ball. So I took my time for about 20 mins asking for the very basics of these things and rewarding even the most minimal response (literally - our back steps were about 1 inch he was so nervous and tense). Then, I spent 10 minutes just standing. Hanging out, talking to the owner, letting him just breathe. Then we set off and tried again very gently. As he'd had a chance to down-regulate, he could suddenly do all of these things with feather-light asks from my reins. In a rope halter. Mind you, this horse normally goes in a 6 inch shank (we're getting him out of that asap). Notice I didn't escalate pressure. I didn't get after him. If anything, i did "too little" by most training standards. I "rewarded" the "wrong answer" by letting him stand after what most would consider him being pushy and rude.
But lo and behold, by the end of the lesson he was a soft, quiet, and willing partner. So next time you're working with a horse who isn't listening, I'd encourage you to ask yourself "is this horse dull? Or is he just so worried he's shutting down?" You might be surprised by the answer.