I have very little mining experience, but it seems like it takes two perfect hits to dig up whatever you're mining every single time. If you only work out where the perfect hit is as you run out of "health", then you damage the drop instead of getting it. When I've been mining, I get all sorts of ***like copper ore and bone chips, and then when I don't get the item and damage it instead, it's much more common for it to be something rarer.