Getting S's on Junlgers

Can anyone tell me the secret? I go something like 13/2/14 on Rammus and get an A+. not even an S. I know that being present for objectives, KDA, and farm are important, but its just so hard to keep up in farm. I'll typically end a game with 120-150 cs in a 30-40 minute game. Whereas, I go 2/3/21 on Sona and i have 28 CS and never get below S these days.

I feel like I really need to focus on farming if I want to get level 7 masteries on junglers, and my team suffers for it. Riot needs to lower the bar a smidge on CS qualifications for the jungler role. For example, I've been playing Amumu regularly since season 3, So I know his kit backwards and forwards. But I got mastery level 7 on Jax and Warwick almost instantly because they just farm so much more efficiently, and I'm showered with S's every game.

Any ideas?

and please spare me the "just get better" response. That's something we all already know. I'm loads "better" on Amumu than Jax and I've had level 7 mastery for months on Jax already.