Why didn't I get an S?

Ok hi, first time posting in forums. I'm FIRED UP.

So back in February I got four S ranks in a row on Ashe, enough to get me through M5 and almost M6. Since then I have been trying to get my final S. Today me and a friend duo queued and decided to go hard and try and get me that S. One game I went 6/1/2 with 100 CS at 16 min and got an S-. The following game is the one in question. I went 14/1/6 with 106 CS at 20 min and got an A+. I got a triple kill and a quad kill. Me and said friend 2v4 them and had some wild outplays in bot lane. We even looked a bit into other players and learned the midlane, who was in both the triple and quad kill plays, was Platinum 3. Its not like he bought the account either because he is active in the ranked community.

TL;DR I did fantastic against a plat 3 player as bronze 2 and think i deserved that last S what happened where did it go.