Thought support mastery system was fixed...

only 6 cs in a 20 min game? That is why your grade is low. They take many things into consideration: KDA (yours is good), wards placed, cs, and objective participation (dragons, towers, Baron etc) so you will really only get an S if you perform in all aspects.

Others have said that you have to forsake the game at times to get a higher grade and hope it doesnt cause problems.

Wards placed, wards cleared, objective participation, kill participation are all other things that contribute that you may have been lacking in.

You need one CS per minute you perfect KDR is cool, but it' less than 50% participation. How many ward were place and cleared? How many objectives did you take? And looking at your build, your healing and damage output was too low to count.

"Not enough cs" Hehe xD

actually you need to ward at least once a minute to even try to get s on a support.