Fortune Enchantments are not working on pickaxes

Whenever I mined diamonds with my fortune 3 iron pickaxe, they kept dropping only one diamond. I mined a Total of 16 diamond blocks, and received only 16 diamonds. I figured it might be something to do with luck, but then I tested it on another world. I mined 12 diamonds each with a fortune 1, 2 and 3 diamond pickaxe, but they kept dropping on 1 diamond per block.