Fiora Bug with E and Ultimate

Hello im a Fiora main and sometimes you E do the first auto for the slow, then ult and you wont proc a vital with the crit hit on your Ult. I dont know why but i think should proc the vital and it really sucks, i missed many kills because of that. I dont know what im doing in specific besides E basis Ult basic and sometimes there isnt a proc. Fix dat please.