Can't get an S

I've been maining Jhin now for quite a time because i think he's just an awesome champion to play. In the beginning, I was always fed and even got an S sometimes. One time, when I was Jhin Mastery 2, i got an S+ with 11/2/7 and 107 cs. Now that i got him Mastery 5, I never got an S, even when I was fed. I just got an A+ with 14/2/19 and 220 cs and i wonder how this is possible. Do you have any advices how to improve my grade? Thanks
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