Most of us will have one of those rides where the horse seems to be full of energy and impulsion but never actually seems to go anywhere and everything we ask seems to be a huge effort. This is frequently called stuffyness. It happens to all of us and is just one of those things that happens when training horses.
The best way to combat this is to scrap whatever exercise you are trying to do and just push on until you get engaged forward movement. Be this in walk, trot, or canter. It will feel uncomfortable, and probably unbalanced but the most important thing is to get that forwards movement. If your walk is stuffy go for a big trot, if the trot is stuffy go for a bigger trot or a canter, and if the canter is stuffy push on for a ground covering energetic canter instead. Then once your horse is powering on you can go back to the exercise you were doing to begin with. Well balanced horses often get stuffy in very annoying ways as they can canter almost on the spot with ease! So kick on, go into a light seat and keep going until you feel the hind end powering onwards.