Getting S+

I have been getting great kda's and well perfomences with Yasuo and Zed in-game and I have been trying to get S for them but for some reason I have not gotten one. I don't understand. I have been getting well farm and great kda's and towers and winning the games but I have not gotten an S. I do not know if this is a bug or if not, then I would like some suggestions as well. Thank You.