Had a friend in the same situation; played Ahri Mid, couldn't get an S no matter what their KDA was, got pissed every time I or anyone else did. Along with the usual (CS, objectives, KDA, wards) what seemed to hold their score back was damage dealt to Champions. They always played cautiously, rarely not having a full mana bar in lane, running when a fight went south, and usually had just a bit more damage output than your average tank. Meanwhile someone else tries, goes 8/5/4, and gets an S with about 10k more damage than the next highest. See if you can get away with more damage output and more risks; it doesn't take much to play Ahri safely, the difficult part is getting closer to the fight to increase damage without dying.