Crafting?

can someone explain the whole crafting thing to me? i know you are allowed 4 chests per month and you can get key fragments from random matches. but what i don't understand is that i have gotten several S ranks this month but it hasn't rewarded me with a single chest (albeit they were all with the same champion at different times.)but i got 1 with this champion when the crafting thing first started but have not received a single chest since...