I've only ever heard an honest reason once - "because I'm skint."
"There was a streak left on a window the second last clean". "Which window?" "The front bay side window," pointing to closest one to the front door, the one we would notice when collecting on the Friday.
"We are having new windows fitted. We will call you when they are replaced." Still waiting for the call 12 years later and the windows were never replaced.
"We have just got a new puppy and we don't want you frightening him with your brush." The dog is now 8 years old.
After 4 years of cleaning a property we cleaned on the one day she was in. "Don't you dry the windows after you have washed them?" "No, we use deionised water and the windows will dry perfectly. This is the way we have cleaned them for the last 4 years and you have always commended us on the clean quality when we collected the money." "Well I'm not happy about the way you clean them." Got a text cancelling our services from him stating he has found a window cleaner who has cleaned their windows properly."
"Your son was rude to me." "No, he doesn't converse well with people so he tends to say very little until he gets to know you." "He didn't dry my back door off last time." "I'm sorry to loose you and thank you for your custom over the years. Goodbye."
"Aren't you going to just do them and not your son?" "No."
"You charge me £5 to clean three windows. My husband and my friends think that's extortion." "I have to work off step ladders to clean your back bedroom window over your extension. It takes me 20 minutes to unpack, clean and pack away which is time that has to be accounted for." "Its only 3 windows and I'm not paying you £5." (She agreed and paid the price for over a year.) "OK, that's on your conscious not mine."
TBH I don't bother asking these days. In the early days when I was younger I used to take the loss of a customer very personally; now I don't. I'm never rude to them and always clean to the best of my ability. If they cancel I don't hold a grudge and will always stop and talk to them if I see them in the street. In our area everybody knows everybody. News spreads and bad news spreads like wild fire.