why do i never seem to get key fragments

I've not gotten many either, Don't think it's a bug because I did get 1.

I've gotten 3 or 4 since it went live. I think it's working right, but it would help if we knew how it worked. I believe it's just a chance at the end of the game, with the chance being higher if you win.

I'm swimming in chests but got no key fragments to open them.

You get the chance to get a key fragment if you win. However the chance of getting them declines the more wins you have (over a month). Riot said they did this so that less engaged players also get a chance to get them.