Thanks for all the messages guys!
To answer most of what you guys have said. I used a brand new rubber that day Unger soft works like a treat no problems before. The windows are perfectly clean, no grease no buildup i asked if they clean inbetween me going there they said they would not dare do that! The only thing i have noticed is that on 2 conservatory windows the gutter leaks water onto them and that causes white dot stains on the glass but i dont think that is related as those windows are better and easier to squeegee than the ones im struggling with!
I definitely had enough soap on my squeegee as i made sure this time round. I will try and scrape 1 pane of glass next time and see if that makes a difference, other than that i am completely stumped.
One thing it could be is as you say is aftermarket coating Rainex(never heard of that before though). Will try to add a lot more ecover next time on this customer to see if it makes a change.
This isnt the usual problem you get with squeegee drag on windows guys, this is so bad on 1 window i almost considering cleaning the entire window with a microfibre as it was almost impossible to clean it.
How much soap do you guys use? I usually fill my squeeze my ecover to cover most of the bottom of an unger 20L bucket i then fill to 10L, is that enough soap or too much? Usually i can last a whole day on about 5L worth and its still really soapy and works like a treat!
If this problem does persist i will have to just clean their windows WFP as the job is worth £9 discounted from £12(good customer of mine) and it takes me about 20 minutes tops but this time round it took me 40 minutes!!!