[Gameplay] Fiora ult on Mordekaiser.

So I just recently got fiora, but I made sure to do a good amount of research so I knew what her ranges were. Then I played a normals with my friends, and during every teamfight, I kept on continually trying to ult him. But for some reason it wouldn't go off. There was a point where I was actually trying to pick him off but my ult just wouldn't work. I know that it was working on other people in the game though because I could continually use it on their top lane garen.