Rito Pls

So i don't understand how this works please help,i have had 2 key fragments and 2 chests forever I've played 15 games and i don't understand,why aren't i getting key fragments? I have played 15 and 0 of anything chest or key fragment in those games I've got at least a S- 5 times and i still haven't gotten a chest nor a fragment. Why?