Today I struggled with a 5-year-old Oldenburger, let's call him Blackie, who just wouldn't canter on the left lead. I tried all the obvious solutions; giving the aid on the circle coming to the wall, giving the aid in the corner, saying "canter" in my lunging voice, being careful to let my inside hand forward as to not block his inside hind leg, .... And Blackie just kept answering with a canter on the outside lead, pretty annoyed at this incessant nagging - after all, he was cantering, wasn't he!? We took a break, worked on some transitions, tried again. But no.
So I let it be, feeling pretty stupid and thinking that I shouldn't press on since I obviously wasn't capable of getting what I needed, and I didn't want the problem to escalate without having a solution ready. Then S suddenly stood in the corner. I halted and explained my problem. She nodded and told me to trot left on the circle. Rising trot, bend him slightly to the outside, inside leg on, bend him slightly to the inside, sit the trot, canter aid. And oh wonder, we had left lead canter.
"See, it's easy", said S with a grin and left.