The excuse you accept repeatedly becomes the service you can expect.

Repeatedly letting something pass is itself a message. Each accepted excuse tells the other side where the line actually sits, and after a while that becomes the level of service on offer, arrived at without either of you ever discussing it. Nothing about this requires bad faith. People simply calibrate to what gets a reaction.

Name the standard once, plainly and without heat, the next time something falls short. Say what you expected and what you would like instead. If it keeps happening after that, you are choosing whether to accept it, which is a better position than not having said anything.