That little 2" x 1" square of cement from the side of the window frame has NOTHING to do with the fact that the customer hasn't paid for their window-cleaning service. They didn't know it was there anyway.
They havent paid you because they either: a) dont care, b) forgot, c) didnt know youve been, d) are away, e) dont care, f) too many other things on their mind to worry about, afterall you are only the window cleaner, g) dont care, h) forgot and dont care.