Well

The first wish wasn't just to turn more female. It was more female in a specific sort of way. A second wish which makes them female in a different way would contradict that. It would be like having a white branch, wishing for it to be light blue, then wishing for it to be dark blue. You'd be making it "more blue" than it was after the first wish, but you'd still be contradicting the first wish, since once you specified in what way it was blue, you can't change that.