Are you eligible for Hextech loot, as in, have you been punished recently? You should be notified after a punishment that you're ineligible for Hextech loot if that was the case anyway. Have you been receiving key fragments?
Are you positive you have chests available to earn and aren't misreading what the chest slots mean? In your profile main page, gold chests mean you can earn one (4 slots max) and greyed out slots mean you have to recharge it first (you charge one a week).
https://support.riotgames.com/hc/en-us/article_attachments/203324116/ChestLeft.png
Also, you are the one that earned the S, yes? A premade ally can earn chests for the champion you're playing if they get an S, but only if they are in your premade. Once you earn a chest for a champion, in your profile on the champion page it'll have a gold border, and the normally greyed out chest while hovering them will be gold.
Other than that, I'm unsure what legitimate reasons may have prevented you from earning one (just trying to rule out all non-bugs first). You can get more info here; https://support.riotgames.com/hc/en-us/articles/207884233-Hextech-Crafting-Guide
Let me know if any of that may have caused it. Otherwise, you could submit a Support Ticket and request they look into your case for you.