HUGE HEXTECH GLITCH... GETTING FREE CHESTS + KEYS (misleading title)

Did you get the S ranks on champion that you own? If not then that is the reason as to why you didn't get a chest.

Second part: Did you win? If yes then you most likely have Key fragments which can be combined into keys. Three fragments = One key.

Hope this helps!

I've played many games where my teammates have gotten S- S and S+ but I haven't received any chests. Why?

You either played a free champ or you already got a chest with that champ.

Keys are somewhat rare, they become even rarer if you already got keys this month.