GAMEPLAY NUNU ULT BUGGED

I was recently playing Nunu on the ARAM map, when i found that i couldn't reclick his ult in order to activate it early. This happened a multitude of times, and prevented me from getting a few kills in teamfights and potentially winning the game. After the ARAM was over, which we lost, i tested it out in a self created bot match. I could not reclick my r to stop the channel. it appeared that while i was channelling, the ability went to the cooldown instantly, and therefore, i couldn't reactivate it due to it being unavailable to click. Thanks.