I have been doing a house for a couple of years with a similar sounding problem.
It feels like when your scrubbing the windows they have a greasy film on, i can scrub and scrub but when the windows start to dry they sort off mist over and leave streaks in some windows, its mad as its the only house in over 200 i do that does it, customer said she has always had problems getting the windows clean before i came along.
It would be good to hear if others have had the same problem as it does my head in, customer is happy but i ain't!!