I'm assuming you did it using wfp, if so then the questions to ask are:
Have they ever had it done like this before?
Did their last WC do it trad?
Why are they unhappy about the clean, it's often a case of they see the windows left wet and they thing you've done half a job, they rarely wait until the windows have dried spotless!
Who's complained? Is it one of the commitee or just a member commenting.
Go there have a look at them, tell them that you always come back to check after first cleans to make sure that you are happy with the result and to check that the customer is also happy.