Good luck with that.
Unfortunately, with trad cleaning there are so many variables that can affect the end result it would be foolish to try to suggest why you are getting streaks.
But heres a list of things that can influence it:
How much water you have on the mop.
How new/good the blade on your squeegee is.
The angle that you approach the window (how you're stood, the way you sweep the blade across the glass)
How quick the water is drying (sufficient water, sunny, windy)
What chemical you are using.
Your technique (covers everything really, but it has to be said)
How dirty the windows are.
Proficient with both hands?
Dry scrims.
Clean scrims.
Just a few to be going on with.
HTH
Matt