Ever since the update to minecraft I have found some blocks (cobblestone,iron, and others) that can only be mined from certain angles. I can be standing on, or even next to it, but when I swing my axe, hoe, shovel or other tool it acts as if nothing is there.

I found when I go to another side i can get it. For example I may not be able to mine it from being on top, but if I manage to go down a level and try to mine it from the side I may be able to get it.