On slow horses, often times the rider makes it worse by nagging the horse.
Same with, on hot horses, the rider makes it worse by taking the leg completely off.
When you are riding a horse who tends to be behind the aids, use very few aids, very sparingly. Many slower horses respond well to more training from seat aids. A slow, behind the aids horse is actually (I find) more sensitive than a horse who tends to overreact to the aids or is "hot". On a slower horse a rider tends to try to aid every stride to get more from the horse - which annoys them or makes them a bit angry and then the nice ones simply ignore you, but most of them will stop and rear (eventually, at least). Hot horses, as soon as they accept your leg, are more apt to be totally chill with you having it there and giving aids with it.
So, give the horse a small aid, the one you want him to respond to, then after no reaction, give him a kick, and if that still does not illicit the response you want, make him wish he had responded to the first aid. Then take everything off, force your heels down and legs off and pat him, if he stops then wallop him. Then ask him to come back, and repeat. If you don't feel comfortable doing this, or don't have the timing (the time between the first aid and the kick and the proverbial "beating" is less than a stride), get your coach to hop on to get him tuned in, and then be very diligent to keep your leg off of the horse unless you are specifically asking for something.
It will only take a few rides for this to sink in, but then you must maintain it every day.
They say money doesn't buy happiness -- well happiness doesn't buy horses!