Yeah, I have noticed that when you need something it immediately stops dropping the thing that you need and it becomes extremely rare, as if this will encourage us to play more. Your Living Wood example is the best example, whenever you need it, everything stops dropping it. I chopped 20 trees completely down to get only 2 Living Wood once, and in that time frame (2 weeks) only got a drop from the mill once. The sudden scarce drops only frustrates us, but as I am a programmer (not of this game), I ensure you it is not a bug, it is intentionally written into the system.