Does it mean the amount of drones a hub is handling is too much? Its at 9/20. Or does it mean there aren't enough drones? Confused. Googling and searching here has not helped.
Not enough drones for all the work they have to do. It may flactuate if they have to load/unload a ship or carry building materials. But if there is no ship nearby and nothing being built you should consider adding more drones to this hub or build a second one overlapping.