You have to adopt the old addage..
"You can't win 'em all"
We know that we always try to do a good job of cleaning and like you, have had the rare customer who is never satisfied.
We've actually let a couple go over the years, to allow them to find someone 'better', and believe it or not we've had calls from them asking if it would be, 'uhhmm', possible to 'huh hum' return.
We don't go back.
I had a garage owner ring up the other day, suggesting that our cleaners can't have turned up to clean at night....and if they did their work was atrocious.
I said we'd be back that night to put things right for no cost

and I have to say, also at the request of the cleaners who wanted to point out some issues.
To cut a long story short, I was able to go back to the owner and report that the fact that the premises never looked brilliant was that we only spent a total of 2 hours there per fortnight, that his entrance door mats and other dirt trap mats were completely useless and the staff actually dreaded going there because it always felt like a lost cause.
In fairness to our staff, they clean dirtier places, just more often and weve never received any other complaints about their work.
I also pointed out to the owner that it had been his sons decision not to have the premises cleaned more often as was our suggestion.
He actually thanked me for the feedback which I always try to give in these situations.
We cleaning companies 'mostly' do a good job and need to stand up for ourselves when being unfairly criticised.
Blimey....rant over

Les