If the window has been polished and is free of contaminants then I'd say mechanical issue. When clean and dry, put you hand in a freezer bag and drag it over the windscreen. If it feels smooth and the same all over then wiper arms or wipers at fault. If you can feel a drag on the glass at some point likelihood is that there is still contamination on the glass
I that's the case, wash and dry it. Then use an aggressive clay and lube. Afterwards use ironx to remove any metallic contamination. Clean once again and then polish with proper glass polishing kit like www.glasspolishshop.com/windscreen_wiper_blade_scratch_polishing_kit
Clean with ipa once done.
If that doesn't work, a cracked windscreen gets replaced for 75 quid