Your horse can only communicate his distress or discomfort to you via his behaviour. It is very unlikely that he is being “naughty” by not cantering on the left lead when he will do so on the right lead. Horses, in general, do not wake up in the morning thinking of ways to wind you up.
If your horse cannot canter on either leg, then he is most likely confused about the canter aid, and will need more training in order to help him to understand. But if he can canter on one leg but not the other, the problem is most likely to be physical.
Our horses, like us, can be stronger on one side than the other, so it is easier to pick up one canter than the other. Or there could be weakness or pain that is preventing him from picking up the correct lead.
Equally, it is worth checking with yourself that you are asking for the canter aid in the correct way on both reins and are not inadvertently confusing him.
Start by watching your horse walk and trot away and towards you in hand, and see if you can see any difference in movement between the right and the left side. Carrot stretches to both the left and right are a good way to see any imbalance between the two sides, making sure you stay safe while performing them. It can be advisable to seek professional advice, either a physiotherapist or similar, will be able to assess and treat your horse. They should be able to offer exercises to help you and your horse.
Once you are confidence that your horse is physically able to carry out what you are asking, you should find that he is happy to canter on both leads. There may be some initial reluctance as your horse may remember that it used to be uncomfortable, but this should soon pass, as he realises that he is now capable of cantering on the left and the right.