you will not get granted a chest after getting S- or higher...
I have 1 chest available to get and the next chest in 4 days , however I was not able to get it even when my premade gets S- or higher [slayer-jinx-unamused] here screenshot
I have 1 chest available to get and the next chest in 4 days , however I was not able to get it even when my premade gets S- or higher [slayer-jinx-unamused] here screenshot
You probably have earned a chest on that champion before.
I just got an S- on Lux and got a box. Looks like you've already got a box on Ryze.