Fortune enchanted pickaxes do not yield multiple drops of iron or gold ore. I had started spelunking in survival with the intention of acquiring a lot of iron for a project. My diamond pick with 'Fortune III' kept giving me single drops of iron, same for gold. I decided to go into creative to test this.

I set up large rows of every type of ore that typically would only drop 1 material each (Coal, iron, gold, emerald, diamond). I then used a wide array of different pickaxes, with different levels of the fortune enchantment, to see how much each block was dropping. Every pick worked fine on their correlating blocks. Diamonds, coal and emeralds were dropping by the motherlodes. The iron and gold however, kept dropping only 1 at a time. 

I haven't tested other tools or resources as the iron is my only immediate issue but it's possible this problem is affecting more than just iron & gold ore. Please fix soon ~♥