Ex - some more thoughts . . .
If you're getting the marks in the same place every time, like a diagonal mark in a top corner, you're maybe not going smoothly enough and the rubber is flexing on the sharp turn, if you know what I mean. Maybe you're pressing the squeegee too hard on the glass or gripping too hard, try an ErgoTec handle from Unger, it's really only your thumb and forefinger that are doing the business, the other three fingers are just loose on the rest of the handle - as Terry mentioned, it's all in the wrist action. Ooo Er Missus!
Is there a chance that the channel has a slight bend in it? Look along it to see if there's a slight curve.
Have a practise with a bit more soap than you normally use, but squeeze as much OFF the applicator as you can. This will give you more slip, but less suds to get in the way.
Failing all the suggestions you've had so far, have you bumped into any friendly looking window cleaners who would be willing to give you a "Master Class" - ask them to watch what you're doing and see if they can figure it out. Or ask them to show you how they do it. Just tell them you're not long on the tools and could do with being pointed in the right direction. At worst, they'll only break your legs for being on their patch
Don't worry, one day everything clicks into place and you'll wonder how you could ever have had problems - trust me.
All the best, and remember - if you don't solve your problems you're not learning.
