This has to be a bug.... I can't get an S rank, at all...

So I am a Lvl 5 mastery kennen top/mid main, and have been trying to get an S or S- with him for a few weeks. I have positive score with him almost every time, my best being 15/1/14 at the end of a game. I usually end up with a creep score of about 150-200. That game I had 203. I also hit towers, dragon, buffs, had about 25% of my teams kills, and had been making good plays the whole game. I had a full party that game also. Got 134+ added to my score at the end. Now as I said, I cant get an S. I use wards, ping, etc. Ive been on forums asking what Im doing wrong, but I still dont see any improvement in score adter the first time, which got me an A+. I had at least 300-400 score on some games, so why dont I get an S? Nobody really plays him so it cant be the % of players in that same pos a,d champ. Can someone like a riot staffer help me please! D: ~Gardaen Patch 6.11 Bronze 3 Kennen Main