key fragmantaion dropping system

It has not stopped working. Whenever you get a key fragment, the drop rate for the next key fragment is lowered, and this continues to happen every single time you get one. The drop rate does reset approximately 30 days after you get your first key fragment at the original drop rate.

Feels like I'm not getting many key fragments anymore. What gives?

The drop rate for key fragments is a curve, not a line. With each fragment you earn, the chance to earn your next one goes down slightly. This number resets roughly every thirty days after you earn your first key fragment.

We wanted to give players who didn't play as frequently a good drop rate for keys, without making it feel like farming was mandatory if you wanted keys.

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