I do a quick wave to most passing cyclists, and usually get a response unless they have aero bars equipped. I don't know if it is just the riders or the ride intensity indicated by those bars, but aero bars usually signal I am not going to get a response. Out where I ride(rural back roads mostly) I even get a wave from half the passing vehicles, and try to respond in kind. The country habit of waving at those you pass is a hard habit to kick.