Several matches now have not received my free chests

Hi,

Over the past couple weeks I've been trying to earn chests, so have been playing a number of champions that i own with which I haven't previously received chests.

With a couple of these champs (that i can remember volibear and gnar) either myself or a teammate has earnt an S grade but I haven't received my chest for this, have been trying to do this before my slots filled up but its at 4 now and haven't received anything so not sure if there's an issue or not.

Are chests supposed to be earnt every time you get an S with an owned champ or is it still down to luck even with the S whether it spawns or not?