There has been a lot of reports of a bug that causes rocks to not be mined. The rocks would be mined normally in the beginning but as you progress the pickaxes would start to hit the rocks without dealing damage.
This bug has now been fixed.
You might be wondering what caused this bug. Well, it was the "It's Hammer Time" talent upgrade. When a pickaxe would spawn as a hammer, which is a 0.4% chance, the collider would change. The next time this pickaxe would spawn, the collider would not reset, which means it would be a normal pickaxe with a wrong collider. This explains why it takes some time before this bug is noticeable and why it just get's worse and worse as you play.