Potty Probs?

Hi all, I'm a in home boarding sitter and across the board, all the small dogs we've watched have not been potty trained. I'm not talking, in a strange new environment so they have a stress accident- i'm talking will walk them, take them out, watch them like a hawk and they'll pee & poop the first thing back in the house. I'm so frustrated and understand that I am a dog sitter, but the larger dogs we watch (literally its the big vs little here) are ALL potty trained and have never had an accident.

My question is, how do you talk to the owner when this happens? Do you just politely say: "we've been having trouble with buddy going potty inside, any tips to make sure he goes outside" or is it a hard no for watching them again? I feel a lot of my clients take advantage of our kindness, and deliberately do not disclose that the dog isn't house trained- yes, I ask, yes, they tell me "of course i just take her out twice a day". Not to mention, potty cleaner is EXPENSIVE.

Sincerely, I rent my apartment and they're ruining the floors.