If it's downstairs windows iv noticed on a huge amount of houses have a very slight scaling of the glass, caused by rain water hitting the sill and spraying on the windows. It also has other causes, watering garden etc and can only be noticed this time of year in the low sun.
I cleaned a house this week inside and out, the sun was shining through the living room window and I noticed this issue. My first thought was I'd done a rubbish job with wfp so recleaned that window, dried very quick in the sun, still didn't look great.
So I trad cleaned it, still didn't come up well. In the end had to descale it then reclean and it looked great.
Most downstairs windows will build up a slight limescale issue over time, some more then others depending on window location.
the low sun this time of year will show up even the most miniscule of blemishes and especially fine scaling on the glass.
Glass is never truly 'spotless', in the right light conditions you will always find some sort of blemish. Windows get battered by wind, rain, sun, birds, cats, dogs, insects...
Explain all this to the customer then offer a free courtesy reclean.
But hey you may of just had an off day and done a terrible job as well lol, even more reason to offer the free clean.