Only getting key fragments

You can only earn one chest per champion per season. Once you earn a chest with a champion, you will be unable to earn any more chests while playing that champion until next season.

If that doesn't apply to you, then you can check out this guide that has other reasons as to why you may not be earning chests.

https://support.riotgames.com/hc/en-us/articles/207884233-Hextech-Crafting-Guide

Maybe you're just not earning S ranks? You only get chests from getting S-, S, or S+ once on a single champion per-season. So if you get a S rank on Zac, you get a chest, any S ranks following you don't get a chest until next season starts. You earn Key Fragments (randomly) after you win matches regardless of your after-match grade, and will only get them from S-, S, and S+ if you've already gotten the chest for that champion. If you don't own the champion you won't get a chest or a key.

Chests can only be earned by S,S- and S+ after a game by you or someone in your premade group(if you do one). But once you earn one on a champion you can't get another for that champion till the next season.

You can only get chests from one champ. I believe the reset every year. You can find out which champs you got them with by going to your champ list and hovering over their picture. If their is a silhouette of the chest, that means you didnt get the chest with them.