I try to use his right leg not left until he understands how! My pony didn't this I got him to go right leg 5 times then left leg 5 times until he gets it!
Mine did the same, I practice on really small fences a simple exercise: you focus on the canter and give him the best aides you can, and if he land on the wrong leg you just push his shoulder out and keep the canter. After a few attempts he should understand and then you just have to reward him when his right!
I would try leaning to the left over the fence or try and bring your left leg back a bit, not meaning to do with all the time just to help him understand better x