Driller Bots will only mine if the mine output is not full. If I recall, each mine can hold 6 ore, 3 on each side of the entry. When starting the game before you have storage, the Metal processors may become full and the ore will back up all the way to the mines. This can also occur if your storage is full. An easy fix is to slightly increase the allowed number of finished goods, like repair parts or semi-conductors. Of course those need storage space too.