I have not seen this happen yet, but if four ships are returning then it is possible they could surround the site while there is a ship there and none of them will ever move.
Can see this effect in the game at 19/10/18 09:49
here it is actually one bot moving back and forth on top of the shipyard (due to #11) that keeps any other ships from moving.
I have not seen this happen yet, but if four ships are returning then it is possible they could surround the site while there is a ship there and none of them will ever move.