[GAMEPLAY] Tahm's Devour is literally just not going off

It's happened like 3 times in my last two games. I've had 3 stacks on an enemy champion. I try to use devour and it literally just does not go off. I don't exactly know HOW it's happening or why, I just know it is. Below is the recourding at 10 minutes in you can see me fighting Aatrox and Morg. Morg did NOT shield him as she's oom and obviously there's no giant shield around him.

http://www.twitch.tv/dot_eleven/v/11479623

PS. I apologize if it's a bit loud and also that my voice is annoying, I hate hearing my voice when it's recorded.