Well, he cut some of Kuma. But not completely.
And through that he understood that he wasn't made of flesh and bones.
So maybe he wasn't paying attention to the breath of all things, which makes sense he was exhausted. And because he was surprised it was steel, that also serves to prove that.
You know, I was going to say that the other option was that he used it and just failed. But that clearly doesn't make any sense.
He probably used back in pre skip by repeating the same feeling in terms of its practical/bodily implication. And sometimes that will obviously fail.
Post skip he hasn't fail to cut anything. So perhaps, he mastered it. Makes sense since now he's conscious of what haki is.
Edited by TrolonoaZoro, 27 October 2016 - 10:16 AM.