Why am I not getting a chest after I get an s.

So I've been playing a lot of lee lately and recently I got an s- with him. I ended up not getting a chest with him. I thought if you got any type of s with a champion you would end the game with a chest. This isn't the only time this has happened ether. This same morning I got a s with some other champion and didn't get a chest. I also made sure to check that I still had chest available (I still have 4 chest available). For some reason I haven't been able to get chest for like several months and I was just wondering if there is something wrong with my game or is this just a bug.