Hextech Crafting

So .. I got the hextech crafting ideea ,.. i got 5 key fregments and got 1 key But .. When i got LvL 5 Mastery Level on a champion shouldn't i get a hextech box ?? I got lvl 5 mastery on Mundo .. and i haven't got a Box .. is There a problem or i understand this thing wrong ? Need some help . . :D .-.