Even if your horse does understand what you mean with your aids, it is possible he’s not very eager to react. It happens when you give aids constantly, without demanding an answer. Or when he’s bored.
Think about it: how would you be if you have to go into a sand school every day, walking round after round, doing the same all the time.
It is very important to motivate a horse, keep him interested in what you have to offer, so he’s happy to go out and play with you. And the way to do it is to vary his training. Don’t go in the school every day. Go out for a trail ride. If you haven’t got the control to do so safely on your own, go in a group. Not possible to go out, due to weather or traffic? Vary the work in the school. Lunge him, walk, trot and canter over poles. If you dare, do some jumping. Put pylons or barrels in the school and circle around them. Use your imagination and make fun, to keep his mind engaged. Learn to do in hand work. Try to become his friend, not just his boss. Create a bond.
Lazy horses can be motivated if you vary the work. Don’t train them for an hour. Do some training for 15 minutes and then do something different. Change the tempi. Think about your aids. If you use them too strong, it might hold him back.
It never helps to use force. It makes them scared instead of obedient. He should enjoy working with you, so make it fun.