I can understand squeaky's problem with customers trying to miss a month here and there. They sometimes do it to me and I always accomodate them. 
Although some on here say they won't let customers run their business, the customers 
are their business. 
No customers-no business and unlike some folk on here most WC's haven't got an unending supply of people begging to become our customers.  :

If we are all being honest,  lets face it the every month clean is a bit of a scam isn't it? 

..... and we can't blame some customers for tumbling it. 
I have been to customers today who were cleaned a month ago and the windows looked perfect. 
I personally think that every two months is plenty to keep windows clean in the modern smoke free environment we live nowadays and I think a lot of customers probably agree with me. 
It's unfortunate but that's life. 
I haven't cleaned my back windows for three months  

 and they just look like they need it now.  
