[Game] Annie Unable to cancel Tibbers?

As an Annie main this season, I'm finding it very difficult to play her right now. I'm having a sliiiight problem with her ult. Almost every time I go to cast it, it will not let me cancel it. I'll click to cast it way outside her cast range and it will not stop me walking. There have been times where this cost us a team fight or even just cost me a kill in lane. It's been doing this with other champions and their skills that are able to be canceled, but mainly her. I know it doesn't happen EVERY time, but it's starting to become a problem.