Why no S? pt. 2

If you have seen my other discussion this is very similar. I decided to play a game as one of my favorite champs Garen. I needed 1 more S to get mastery 6 on him and I was excited. I knew this game was going well when I was 3/0 ten minutes into the game.eventually got 1 more kill and died. Now i'm 4/1. Got a couple more kills 8/1 now, but then I die again. I knew that if I didn't get more kills I wouldn't get an S. At the end of the game I end up going 18/3/11 with 190 cs and with the new honor system I was the most honored on my team, but soon my smile faded away as I see A- as my grade. Will I ever get mastery 6???