Why you aren't getting much keys
I've seen a lot of people complain about it, but I've found the answer. Riot has put it on a linear chance, meaning that it scales down each time you get one. Once you get a certain number of them, the chances of you getting another gets EXTREMELY low. Appeantly there is a limit, but for right now we don't know what it is. It could be 5% or it could be 0.00000001%. However the good news is it resets every month, so your chance goes back to normal.
I personally would really like for riot to make those numbers visable. Especially the month thing, just like the boxes are. I want to see when my percentage is at its peak to earn, and what that percentage is like they do with the boxes. Source : http://boards.na.leagueoflegends.com/en/c/bug-report/rJ4rQmHA-after-getting-4-chests-key-fragments-no-longer-seem-to-drop-for-a-lot-of-people
Posted here as well to hope a rioter sees this.