ive has this too.a couple of times over the last few months ive had customers saying there is a bit of dirt still on the glass after a bird strike and clean when ive gone back collecting.both times it was upper windows,one of them a very awkward angle window.
nobodys perfect.both customers paid up and said dont worry about it.

i use extreme,sl and supreme brushes.
i always give them an extra scrub and rinse but i think sometimes the marks are greasy and the brush and water dont quite cut it.
its very rare i dont get all the dirt off so i dont worry about it.
