Good quality service/products/installations and happy customers are two different things.
For some, you can mess up a bit and they are still happy. Their standards or expectations are lower.
For others, you can provide perfect quality and they still aren’t happy.
They had higher standards, higher expectations, they are quirky or just hard to please.
To make everyone happy you have to deputize them as the quality control inspector by asking them if they are completely satisfied, inviting them to complain, and being prepared to go out of your way to handle a few difficult people once in a while – while smiling and not being argumentative.
It’s their game, not yours.
Play to win.