Vel'koz glitch

So I played a game with Vel and half way through the game, I couldn't ult anymore. I tried pressing R multiple times, but it didn't work. It wasn't on cooldown because it didn't work for the last half of the game and its not my keyboard either because when I pressed R, the game made that sound when you activate an active item. It only happened to me once but Vel's ult is highly influential in the outcome of a team fight and the whole game. Please fix.