I did a house with relatively small sliding sash windows today. On each pane there was a significant overlap of the squeegee and on the second pass it left streaks due to the drag. Each pane had to be polished with steel wool to make it streak-free.
The mix I use is: four litres of water with a squirt of Unger liquid, squirt of Ecover washing-up liquid and a teaspoon of Easyglide.
What am I doing wrong?
Unger liquid and Ecover? Using washing up liquid defeats the reason for using Unger Liquid. Also, washing up liquid degrades rubber quite quickly.
I tried Easy Glide for a few months once and found it does tend to also smear. (The best place to put easy glide is in with the windscreen wash in your car.)
If you don't get the same problem with other windows then maybe it could be something to do with what the windows were cleaned with previously. Maybe get a good hard surface cleaner and give the glass a once over first using a dirt cheap 99p shop brillo pad to remove anything left on the glass.
Wire wool though? All you need is one little bit of rust and you've scratched the glass with loads of tiny little scatches that you won't see but if the sun shines on it at the right angle, your customer will. Been there, done that, got the letter of complaint and paid for the replacement glass.