Getting an "S"

So, first off I'm an main. I love her abilities and I just can't get enough of her. I'm a Mastery level 6 with her. But the thing that reeeealy bothers me is that sometimes when you play a champion you love, you don't get your S. Like lemme give you an example of my last match with Anivia. I was 14/0/16 and had a CS of 169. I didn't get the time of the match but it was about 34 mins. I was doing pretty good and I was happy about I was in the game. I FINALLY thought after all this time I would get the last S I needed to get Mastery 7.

After I won that match I clicked in eager to get the last shard I needed, but guess, what? No S, not even an S-. I was prompted with an A. But REMEMBER, this was only one occasion, I've done things very similar to this so my question to you Riot is "What do I need to do to get my S???". I'm pretty much done trying to get that Mastery 7 Anivia. Anyone else got something similar??