the key is the first clean, this is what i do on all first cleans, go round all frames, sills and vents first giving them all a right good scrub and rinse making sure all the dirt is rinsed away, then start back at the first window and do all the glass, i always have a high flow rate on first cleans, you want to be rinsing everything away! don't be afraid of vents just make sure on the first clean they are well rinsed and you will never have a problem,