Grading seems unfair.

Anyone got any tips on how to get a lvl6 for an adc? I feel like the Grading System is bugged. before i got a few of my adc's to lvl 5 i got S ranks for everything, Now since i am a lvl 5 its like impossible to get it. Grading Systems seems to be off or something that isnt right with it. idk if anyone else is having the same issue or not.
https://scontent-iad3-1.xx.fbcdn.net/v/t35.0-12/13682383_10205811398518801_283900864_o.jpg?oh=28f0f5be990e21e69d2bf97cfb5ccf4f&oe=578F7404 here is a game i went 20/0/17 and didnt get a S- or s+ but someone else did get an S they went Ex. 2/9/12.. idgi.