I think I know the reason for it. Ships cannot target other ships unless they are on the same sea zone at the start of a turn. However, if the AI keeps moving the ship, there is no way to fight it. It keeps avoiding all ships sent to sink it.
The Italians did that to my Egyptians blocking Khazar and Kiev permanently and Constantinople every other turn. Even when I am not targetting it, it keeps moving so I think that may be the reason.
Bookmarks