What are we supposed to do to get chests?

I've got five keys and haven't gotten a chest in months.

The last chest I got opened to reveal a champion shard. I've got enough IP to unlock any champion I want; so, that's blatantly an insult.

I heard that you only get chests for S ratings. If that's true, that is ridiculous. This is a TEAM game. My score is often impacted by what kind of players I get matched with. Not to mention that S ratings are so picky.