Vayne condemn stuns, but doesn't knock back

I used condemn on a Darius in a fight towards a mid lane wall, but he didn't move. The stun went off and it made a sound like he hit the wall, but he didn't move. He didn't rubber band back either and everyone in the game noticed it too, other than me. It didn't happen any other time throughout the game, and I don't recall if there was any strange interaction between anything. It was the first time it has so obviously happened to me playing Vayne.