I have noticed a few times lately, that when scrubbing away at something that doesn't come straight off (like mini bird mucks for example) the WFP brush can pretty useless.
You can come across a mini bird muck or similar and give it a good old scrub for it not to budge, then as your stood right next to the offending article you simply put your hand out and give it a little nudge with your thumb nail and the seemingly well stuck on "mark" falls off!
The same with them little yellow bug spots, near impossible to shift with the brush yet removed with ease with your nail. I find this with all my brushes, even the tucker dual trim which is by far the best heavy duty scrubber I have. Even the little du-pont hybrid ultimate (aggressive little bugger) suffers the same issue.
I can give a window the best damn scrub its ever had on a first clean, virosol etc etc. Only to go back when dry and find things that are easily removed with a cloth.
The good old T-bar mop is way better at "shifting" things!