One of the biggest culprits of why horses refuse to go forward and have sticky gas pedals is their riders giving them mixed signals. They cue their horse to walk forward, but at the same time, they have a death grip on the reins, pulling the horse’s lips back to his chest. The horse has no idea what he’s supposed to be doing. On one hand, he’s being told to go forward, and on the other he’s being told to stop or back up.
That’s why, when first teaching a human or a horse how to move forward and establish a gas pedal, I make everyone ride on a big, loose rein. In fact, I have riders put one hand on the reins and then place their other hand on their horse’s mane or on the saddle horn to keep them from grabbing onto the reins and micromanaging their horses. It sounds like a simple concept, but you would be surprised how hard people find it to let go of the reins after years of riding that way.