Getting an S in game and not getting a chest

Lately I've had multiple games where I end the game with an S score, but I don't get a chest. I'm almost maxed out on chests available, so this issue is becoming something that is going to cut into the total chests possible to get soon. If there's a requirement other then having chests available to win, and getting an S in a game, please let me know.

For the record, almost all games played these days are in ARAM if that helps any.