Key fragments won't drop!

After a literally 35 games total of winning, and 20 of them with S ranked. I've only had one key fragment dropped out of a week of grinding those games. Most of those games, I spam the same champion, "Lux, Kog'Maw..." and failed to see any key fragments drop at all. It is driving me nuts! I believe this has to be a bug, and on top of this, some of my friends are having the same issue, while some, key fragments are dropping like bird poop!