AS Paul said, scaper on a pole if it is really a problem, some bird poo is hard to shift, seagull poo seems to be the worst
But if it is hard to shift then just give a pre-soak, do another window or two and come back to it.
Sometimes it takes persistant scrubbing, but it does come off.
On windows that haven't been cleaned for a very long time and are thick with dirt, hold the brush off the windows and give them a pre-soak (just a couple at a time) pinch off your water supply and either dip your brush head in your bucket of water & detergent (preferably GG3/Unger liquid etc) and wash over the windows with a high flow rate.
You have to be quick as the detergent is soon diluted down.
But on really grimy stuff it can work well.
I personally have a hand sprayer with a detergant solution on hand for any work where I think a little detergent is needed, and then I just spray the brush head.
Sometimes need it for some ground floor windows too, I just spray onto glass when needed then.
Usually on takeaway smears or lipstick marks where some woman has left an imprint of her lips on the glass
I do lots of shops, so often come across little problem areas.
It takes longer of course, but on windows that are that bad, it takes longer to do them trad too.
Whether you charge extra for that first clean of not is nown to the individual.
If it is a one off clean then rack up the price.
And David...
Tree sap windows are a nightmare with trad too
I hated them!
With WFP I treat in the manner I described above, or even stick my applicator on a pole and scrub the windows with that first.
Some things need detergent to break them down.
Ian