wards?
As a support your letter ranking depends a lot on wards you put down and enemy wards you destroy, looking at some of you previous matches it looks like you need to be doing those 2 things a little more and you can easily get S- or above.
probably not because I got one going 7/8/20 and basically ignored my support items (Skipped Sightstone and Frost queen's)
http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2206903644/226761763?tab=overview This match. 96 wards placed lol didn't help my B