[BUG]Udyr's Bear Stance occasionally doesn't stun enemies.

It's all in the title. I was playing a game yesterday as Udyr (Spirit Guard Skin if it makes any difference) I was going at someone with Bear Stance, they didn't have a Banshee's Veil, or Morgana Bubble, etc etc, and when I tried to stun them, nothing happened, the "You can't stun them yet" indicator popped up and they kept walking around doing things. I couldn't stun them until it went away.

One instance I can think of this happening specificly. I ran up to a Rek'sai, she unburrowed, and I stunned her at the same time, only she didn't get stunned, and the "You can't stun them yet" indicator popped up and she walked away while I was airborne/stunned.

I chased her down and managed to actually stun her, but I noticed this happen to lots of different opponents. :(