How do you get S ranks and chests?

I assumed that you would get chests if you or a teammate got an S rank during a game?

That's 3 games in a row with different champions that I had not received chests beforehand for where teammates got S ranks and I didn't get any chests? (I also haven't gotten any chests for over a month)

Also, I had a game where I was Jungle Nautilus with 2/4/30 with 132 cs. How is that not an S rank?

What exactly are the criteria to getting S ranks and what gives with the chests? Did that change?