This usually happens when they cannot unload because either your ship or your warehouse are full.
If you have more details I'd be happy to take a look and maybe raise a bug report.
So fare there have not been any issues with the cargo drones.
Viewing post in Version 1.20 Public Release comments
It could have been what happened since the warehouse was full for a short while, but it should not have been when they were supposed to reach te warehouse, and there was plenty of space there afterwards, but they never left the 'blocked' status.
Logically, they should have been able to dock there and unload once there was enough empty space.