Cassiopeia, for some reason I can't get an S

Relatively new to LoL and fell in love with Cassiopeia. I have been having fun trying to get level 6+ on my champs that I enjoy. I only play blind norms because I enjoy the randomness.

For the life of me I can't get S- on cass. Does anyone have issues with this on any of their champs?

For example Today I went 18/2/13, many games with players with the exact same numbers receive S's but not little miss Cass. I still have loads of fun but I just don't understand why one of my better champs I can't get above an A. It's just perplexing mainly.

My cs score isn't the best due to mid game I can't waste a lot of mana clearing minions...can this have that much effect on grade?