i've had a couple of windows that just keep spotting even after 3 cleans,what i have just started doing is putting a couple of drops off gg4 on the brush head then squrt the windows just to make them wet then srub the frames and glass,so theres sud's all over.
i do the whole house with just two drops on the brush then go back to the first frame and clean as normal with an extra bit off rinseing just to make sure the suds has gone.
so far it seems to be working and i'll do this on my first time cleans to.