You could have a buildup of polish and whatever contaminants were on the surface prior to applying the polish. In this case, car wax will only add another layer that'll dull eventually. I've never encountered this problem, but I do know there are stone cleaners available that claim to remove stains and soils without damaging the surface or leave behind chemicals.
added: Oh, yeah, what Avenging Knight said.
I've talked with granite salespeople from stores that specialize in countertops. They always recommend a sealer every 12mos-18mos. It's probably overkill, but it's an inexpensive preventive maintenance.