Hi Again,
New builds are the worst - you need to wash the windows down with a degreaser and that should sort them out. I cannot remember the name of the stuff we use, but I will look it up tomorrow for you.
Also, the spotting your getting is probably of the new builds also, this will be grease drawn from under the rubber in the frames, explain this to your clients and tell them it will take probably 4 - 5 cleans before all the grease is drawn-out and nothing can be done about it. It should not take that long. On your next clean, really work on the top of the frame, give it a good wetting and scrubbing, move onto the next. Then do it all again when the first clean is dry, this time, try not to wet the top of the frame, then you should have a spotless window. The next visit after that you will find easier.
Hope this helps
Andrew