Hextech Crafting

The Hextech Crafting system has been out for a good while now, but I'm still confused on one thing. I've been playing on the Summoner's Rift pretty consistently for the past week and a half. Yet for some reason, I have not been able to get a single** key fragment**. I've talked with some of my friends on the subject and they have had trouble getting fragments as well. Is there something that we're missing? Or are key fragments that hard to get? I know that you can only get so many chests within a certain time frame and by getting S ranks in games. But I was under the idea that obtaining key fragments were by random drops. If someone could help me out in what the reason for this is, I'd be very appreciative.