My Ult is not going off with Miss Fortune... Could it be the my attack move setting?

I played Miss Fortune almost every game. Lately, maybe last patch or sometime in there I noticed that her ult doesn't go off all the time. I literally have to stand still for a complete moment and then cast the smart cast "R" and i cannot do a thing until it finishes. If I try to cast it in between a combo and step, you can hear her laugh once and nothing happens, then if I do manage to cast it I cannot touch another key until it is over or it is cut short. So in order to cast her ult, I have to really concentrate on pre-attack and then post-attack move. This is way difficult if you are in the team fights or in a ranked game where this can really count. I am basically just an expensive feeder with her and I get raged on constantly, with MF.

I have smart casting on and I have the attack move set to a mouse key which cuts the animation and I am wondering if that is my problem? If so, well, it works wonders for other champions, but not with MF. Anyone have this problem? Any feedback would be appreciated greatly!