This is very annoying. I get key fragments nonstop but I can't remember the last time a chest actually dropped for me. I guess it's because I main one champ and I've already acquired too many chests with that champ? I get S's all the time but it's almost always on my main champ. Any other reason why this would happen?
You can only earn one chest per champion each season. You have to play other champions to earn more (you can see what you're already earned a chest for in your Profile on the Champion page, they'll have a gold border if they already got one).
Look at your profile to see how many chests you have available. There's a maximum of four and refreshes one about every week. You may earn one chests per champion, per season. Check your collection to see what champions you have already earned one with (will say chest acquired when hovered over and has a gold trim around the champion icon)
I usually get easier chests on week end modes than week day play, keep an eye out for the modes.
