it takes time
Take your time, you can use a concret cleaner prior but it still is not super fast.
Compared to a wand yes it's faster and even, but still if you work to fast you'll defeat the purpose you'll get use to it
elements
It's cheap siding and maybe older then six years find out the manufactor of the siding and how old , is it run off from the roof ? What i think what happen is what a few people already suggested , you reveiled a problem, we spray water and cleaner not paint and the customer...