Amumu ult wont activate

I was recently playing a ranked game as , and during our final teamfight at our nexus turrets, i used my Q to jump into the middle of all the enemies and pressed R, but nothing happened. I kept on pressing R multiple times, i even used my mouse to press the icon on the hud but it just wouldn't activate. This caused us to loose the teamfight and loose the game, which kinda annoys me. And for the record, my ping (ms) was down at 41 so that couldn't have been the cause.