I think American women are, overall, bad at flirting because Americans, despite the hype, tend to be more prudish, especially in their assessments of the moral character of aggressive women.
It would help if we, as a whole, stopped identifying assertive behaviors in women as being a) emasculating of the male, b) dyke-like (as in an unequivocal display of lack of feminine characteristics in a biological female), c) slutty.