Try using a soft scouring pad on the windshield? Possible it's built up some grime and crap on it depending on how old it is. Using a pad that's safe for teflon of course, not a brillo pad unless you want to scratch your windshield

I use Rain-X on my windshield and my wipers are over 2 years old now, and the car has 210k miles on it and I don't think the wiper motors have been replaced.