S rank is this hard?

(Disclaimer: I am very new to the forums/boards so pls give me any tips)

So for the past about 2 weeks I've been grinding out thresh games to hopefully get a mastery lvl 7 or 8. The second game i played after the mastery 6+7 came out, I got an s-. Now i'm finding it incredibly difficult/impossible to get an s-, s, or s+. A very recent game i played really displays my confusion. It was a thresh one for all game, and I ended 7/2/31. If you ask me, that's a pretty good score, considering it wasn't a stomp. But the weird part is I got an a-. Now you might be thinking "It's one for all, it's probs just harder to get an s". But here's where my real confusion sets in. 3 of my thresh teammates got s- and s ranks. One of them went 15/11/16, and got an S. Another went 10/5/26 (a impressive score nonetheless), and got an s-, and the final one who got an s went 13/7/11. Now I'm not saying that these scores are not worthy of an S, but I'm just wondering why I didn't get one. I also was top, a lane thresh does't usually play, so that excuse you guys make doesn't work... Anyways, this is a long arse story so yeah..