Welcome to the forum Mark,
Baked on bird muck, yep, if it's baked on then it's a problem
You can get around it usually though, and thankfully it isn't on every window
Baked on muck you will pre-wet with the pure water and then carry on and clean a few windows (I'm talking upstairs windows of course) when you then come back to it, with any luck it is soft enough to get off with the brush and pure water. Pure water is more agressive than tap water and often then is enough.
If it isn't, then use the plastic edge of the brush, easy enough when you tilt the brush head to one side.
Still to hard and baked on?
Give up with brush, swear profusely, throw brush on floor and attach your Unger 4 inch scraper to a squeegee handle, pop it on the end of your pole and knife the now hated piece of fossilized bird poo of the glass
I've been using WFP for just over 12 months now and so far I HAVEN'T needed to do that, but I do concede it is a possibility
On lower windows it is a simple matter of course.
Regards,
Ian