I am tearing it up on AP shyvanna these days and I never get S or S+ and thus never get a chest. I am having games where I have 4+ KDA and the most farm in the game and I am getting S-. Sometimes I am completely carrying and have no deaths and I'll get an A+.
http://na.op.gg/summoner/userName=always%20blamin%20jg
It just urks me is all.
Go on your profile page, look for the "stats" tab, then find Shyvana.
The stats there will most likely give the best indicator for what you need to work on.
that is due to the fact that some champs are just easier to get good scores on.. and when u care being compared to good scores its harder to get even higher than those.
for example.. even though right now get S or S- on nidalee fairly frequently.
back in early season 7.. when she had a 38-42% winrate.. i was literally getting S+ in like 80-90% of my games
horrendously OP champs like 
are going to be almost impossible to get S+ on becuase EVERYONE is going to do good, and the people who MAIN them are going to be unstoppable.
the stronger your champion in in the curret meta, the HARDER it will be to get an S or S+, becuase it means many more people are likely to have a good score. which raises the bar MUCH higher for said "S".
If all else fails, run some blind picks/norms and play her in an off role like mid or support, and do decently well there. Some champs off-roles are the easiest way to get that S, if you're just trying to level mastery.
I have played 7 games ever with Shyvana and gotten S ratings in my last 2 with her. Janna is a character where I will have insane KDA and it seems impossible to get an S.
