The issue is whether the problem is a provider lacking in basic courtesy/respect/professionalism or a totally inadvertant screw-up (i.e. cell phone problem). My rule is to cut everyone slack for one screw-up. But if there's no apology for the initial screw-up, or if a second problem occurs, they are off the list forever. There are lots of providers out there who are solid professionals. There's absolutely no reason to waste time on the ones who aren't.

Twice I arrived at appointments to no answer on the phone and once at the door.  Both times the ladies were still getting themselves ready for me. I think a hairdryer was the culprit once and the 2nd time I was arriving to a double and the ladies were "busy getting to know each other" and didn't hear the doorbell.  I do not regret at all the persistence required to make the appointments happen after all.