Have you ever had a situation similar to the following happen to you?
You decide you are going to teach your young horse to ground drive. He has never done this before, but he is accustomed to ropes around his feet and legs, so he doesn't mind that touch. He does seem a bit confused at you walking behind him and turning him with the long lines, but after a short session, he seems to roughly get the hang of it, and there was no resistance, so you decide that's a good start and leave it.
You get busy with other horses and other things, and don't get to another ground driving lesson for a week. When you attach the long lines for the second lesson, suddenly the horse seems like a pro! He turns smoothly, can do serpentines, and even halts squarely and quietly at a light touch. You are dumbfounded.... was someone secretly working with your horse that you weren't aware of?! Did he just regain memories of a past life as a cart horse?
I had this exact situation happen to me. Similar ones as well when teaching a horse a new concept. I finally decided that horses take a much longer time to "digest" new information, and their timeline is completely different from ours when it comes to learning and assimilating new knowledge.
Then I heard of the concept of "latent learning" in a podcast on clicker training horses. So when teaching a horse something completely new, such as a brand new trick, or a flying lead change, or the turn on the forehand, you will achieve much more if you give your horse breaks of a few days between lessons, instead of practicing daily.
Most horses trained once or twice a week for a short period learn more between sessions than those taught every day in long lessons.
The horse's brain needs time to process and absorb what it was shown. Then it needs to put that information into the long-term storage section of the brain, where it is archived and can be quickly retrieved by the horse when needed.
On the other hand, humans usually learn in one hour classroom blocks, so we expect to move at a much faster pace than the horse.
On her page, professional horst trainer Stacy Westfall says:
"I love to work with horses that I call ‘next day learners’ or horses that the next day come out and have really absorbed the lesson that was taught the day before. They may not be experts but they seem to have thought about what went on and they are applying the concepts at least."
Slow down!
Think of a horse's timeline of stretching out much longer than your own. I have had a lot of success teaching a horse to pick up a leg on cue (a light tap with the whip) by asking once at the start of our grooming session, then a second time when I am all tacked up and ready to head to the mounting block. Upon finishing our ride, the horse would frequently offer to pick up a leg while untacking and brushing out. It's almost as if the horse had been ruminating on the task over the course of our ride, and decided to show me he's figured it out!
So try not to repeatedly drill new tasks with your horse.... give him time to "digest" the lesson and his learning pace will actually increase.