Where is my chest?

I just played an Aram with Miss Fortune and went 41/13/36 with an +S and didn't get any loot. This is not the first time I have gotten +S and not gotten loot. It's happened on summoner's rift as well. Why doesn't the loot system work? On my profile it says I have 4 chests left that I can receive. I've even had games where other players did a lot worse than me with an A or A- and getting loot while I have S's and get nothing. Am I missing something?