Boredom maybe. Lots of horses have a ton of energy when they are out in a field exploring something new. When they get to the same bland arena they always go to, they lose that excited energy. Don't allow the horse to lost its energy just because it is in the boring arena, rather than outside. Your horse should respond in exactly the same manner, no matter where you are. When you ask for a response, the horse had better give it immediately. If it doesn't, you will need to get after it. Don't nag and keep kicking. You will only make the horse annoyed and even more dead to your aids. Cue the horse. Expect a response. If you don't get it, cue again and give the horse a solid whack with the whip at the same time. Let him respond by going forward. That should solve your problem. It is a respect issue.
Strength is not defined by physical ability. It is determined by your actions and the compassion of your soul.