I try not to let it bother me if somebody doesn't wave. Another thing I thought of was most of the time I recognize people by their bikes not by face. So it could be they don't always recognize you because it is a different bike.
I have also been known to ride my road bike in a regular pair of shorts and a tee shirt even on training rides, so other roadies I see don't see it coming when I blow past them. You know all that she can't be a serious cyclist she isn't in spandex thing.