How do i get an S?

http://matchhistory.na.leagueoflegends.com/en/#match-details/NA1/2210913090/226936523?tab=overview

Played many games as Zyra supp, mid, and even top just trying to get an S flat or S+ in order to finally get her to mastery 7, and at this point its simply just frustrating. No matter how well i do its always an S- minus or below, even though Ive gotten plenty of S+ ranks before i got to this point. The game above is just an example (and keep in mind the morg on the opposite team got an S) , Ive even had many games better than this with no S or higher, someone please give me some advice