Interesting thread.
I've had, I think, 4 complaints in 5 years: 2 the marks were on the inside, 1 was what I thought was a good clean (done with TFR) on a white PVC-framed property, but the glass was not clean when dry, though did clean up nicely on the callback (more later); the 4th I couldn't see what she was on about whether from inside or out
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I use a Superlite with pencil jets on an SLX with a swivel Resi-neck. I did use a Vikan sill brush when I started but, with the Xtel pole, it was like swinging a brick on a bamboo cane. My TDS is always 000, but I live in a soft water area and use both an RO and DI; I change my resin every, say, 18 months. My flow is on 65-ish for a first clean, 54 for maintenance cleans.
I always clean vents on a first clean to get as much dirt out as possible, then regularly thereafter. So it's a wet of the window first, then top frame including vent, then sides and with the swivel on the resi-neck the sill. I don't stress about the sill and no-one has ever commented it's not clean - with the right angle a swivel resi-neck will allow them (and many Veluxes) to be cleaned (Veluxes without collapsing the pole to alter the angle). After i've done the frames I clean the window. I rinse off the glass from the top frame, but only the glass. On most first cleans I return to the first upper storey window and rinse again to make sure that all vent muck has been rinsed away. Ditto on ground storeys. (I charge as much as double for a first clean, so I'm covered. I never price from Google).
On the one clean I had a genuine complaint about I have no idea why it was dirty. It wasn't spotting or streaking or vent drips, but it was dirty (more like lead patination, but def not dry TFR or missed windows). Not all windows were affected and it was a bungalow, so it was easy to see it was "clean" when I finished it originally. I used TFR as there was some algae and the frames were mint when I finished I still have no idea what the residue was, but when I cleaned the affected windows again and let them dry they were perfect.
I'd say that the issue was a combination of the factors already raised. Overspray is a big issue - if there is too much power then the water bounces off the glass. I've not found that brickwork gives me any problems, but the (coloured?) render used on many new houses here in Wales does - it's dreadful. I'd suggest a swivel Resi-neck is a cracking piece of kit and would, as some else has said, suggest you can't clean some windows above connies well without one. It also gets you away from a sill brush. I know nothing of fan jets as i've never (nor would I) use them. Why tolerate a TDS above 000 - due diligence and you know it's not the water. Water does change in TDS out of the tap, so this needs to be monitored, especially in hard water areas. Again to deliver a satisfactory clean vents do need to be done - avoiding the top frame is a cop out in my book. Once done maintenance cleans keep them clean.
Glad your issue is all sorted now.