How do you get a chest?

I thought if someone on your team earns an S rank, you get a chest. As long as you didn't earn a chest with the champ you played already and you didn't get more than 4 chests in a month. Just now I played a PvP rift and someone on my team earned an S-. I played with Garen and since I owned Garen and I previously didn't earn a chest with Garen and I haven't earned any chests this month I was expecting a chest. However, I didn't get any. Do you know whats going on???