[INGAME]Vel'kos ult did not work after 20 mins in game

No idea how to reproduce it.

However about 20 mins in game while trying to steal dragon I was unable to us his ult. Tried to click the icon and press R, the icon lights up but does not cast.

Some background :

Was playing support Vel'kos. Didn't work the whole game after that. Did use it earlier in the game.

Anything else?

Was about 30 mins ago, the game.