Using a drone that's any decent enough size will result into 2 things:
# the drone having issues moving into position to mine the asteroid, it will usually orbit the point it has to sit on. Could be caused by their center of mass being too different from the ship core resulting in unexpected turning circles.
# the drone flying towards the asteroid (sometimes even ramming it) only to proceed going back to its starting position. It constantly loops between going back and forth rapidly without actually salvaging. The drone does seem to aim correctly, just not fire.
Small drones seems to work fine, the bigger ones (60-100m in length) do not.