Ship retraining

Unlike unit retraining, when I try to retrain ships it just doesn't work, for some reason. I have a 21-man Bireme sitting at 100% completed in the recruitment window but nothing happens, turn after turn. I try retraining it in several different ports, with no luck. Any ideas?

it works fine for me. You have to make sure you dock the ship at a port of a city than can build the unit. If the ship is docked and you could build another one of the same type, you just go to Retrain and put it in the drop menu. That's what's working in my game.