Without knowing everything else you did its hard to tell exactly but from what you posted I can tell you this.
The grades you receive are based on how you compare to other people in your region on that particular champ in the role that you are using them.
So Thresh in ADC role is graded differently than a Thresh support.
In your game you got the S- you was 13-2-6......but in the game you got the A u was 7-3-13.......this to me makes sense because there is a big difference between the 2. In the A game u had 6 less kills , 1 more death and 7 more assists.....so if you were Ad thresh in ADC role it makes sense to have a lower grade than the S- game.
For the second A game 21-8-19............the win or loss doesn't affect your grade........only thing i can think of here is that all of the other factors that factor into your grade were probably too low. The grading system uses many factors...not just the kda. Kills, deaths, assists, your cs count, # of wards placed, # of wards killed, Towers you destroy (if you do not get the last hit and a minion kills the tower you will not get credit). Also other objectives like helping kill dragon and baron.
Some tips I use to try to boost my score are.
- If i see my team getting dragon, rift herald, or baron and i am near....i try to make it there even if i just get 1 hit in so that i get credit for helping.
- At end of game whether you are killing the nexus or its pretty clear enemy is about to kill yours......I will drop every ward i have just to get a few extra off.......if im support this will get me 3 from SS a 2 from tricket if i didnt get a sweeper.
