Mystery Chest

Hey there guys. So I've been playing a lot lately and noticed, that I don't get any chests. I know that you can only get one per champ per season, that you have to own the champ, that you have to be premade with people who get a S and that there is a display showing how much chest are available for you right now. I've paid attention to all of it and still got no chests in the last few month - my last chest I got in the week the feature was released. So by now I have 8 keys and no chests... Is this really just random, am I doing still something wrong or is this a bug?

Best wishes