What do i have to do to get an S+!?

I quite enjoy the ranking system implemented into league of legends. Having the game assess you after your play to give you a rank on your gameplay is not only challenging but fulfilling when you finally see that sweet sweet S. But I have an issue; what are the criteria for getting an S? I've had 3 games in the last couple of weeks that, in my opinion, have been S worthy games. But the game says otherwise. Maokai: 19/1/20 with 131 cs (34:57), Maokai: 20/3/11 with 225 cs (43:14) and Ekko 15/2/5 with 144 cs. all A+. tantalizing it was every time, as I felt as a player I was growing but game thought otherwise. None of this would even be something worth mentioning if it weren't for the games that were called A+ games as well. Yorick 1/3/1 with 155cs (21:38), and thresh 2/0/15 with 17 cs.

are the games that I carry away with nearly 40 involvements in kills throughout, the same as those I lose lane to farm the rest of the game just to stay in it? Is there no distinguishable difference between a mid who beats lane by a handful of kills, and a thresh who does nothing but set them? Am I missing some sort of criteria that would more clearly outline how an S can be achieved in each role? Any insight would be greatly appreciated, thanks!