I know you can only get 1 chest per champion per season. I have gotten S or higher on multiple champs(or in premades where my premade partner does) and have not received a chest from the game. S rank on Veigar no chest yet this season, you can also see other champs its given chests on.
Either, you ran out of chests at the time, or there is a bug.
Since you play premades, it's probably b/c you were out of chests at the time.
You can only get one chest per week, or something like that, and can only accumulate up to four at a time to get. Someone else can link the Riot post explaining the rules better.
I have 4 chests open and have gotten S with 2 new players and haven't gotten my chest for either of them and haven't gotten a chest yet this week. Think it is bugged.
