I have several stacks of enemy ships that my ships can not attack. The game just won't let the attack happen. These stacks contain 30 ships and a fleet of 5-7 ships, totaling 35 to 37 ships per stack. These stacks apparently are so full that when the enemy launches a ship from orbit the ship goes to another tile and starts building another stack.
I have a save file and will open a ticket to send the save file to support.