I'm not sure what they were thinking. Admittedly, the abduction/disappearance was a highly unusual situation. It's pretty rare that a woman, her daughter and a young house guest are all abducted, leaving their cars, possessions and no signs of violence.
There was probably a lot of denial going on. Everyone probably assumed since a responsible adult was also missing with the girls that it wasn't a case of the girls running away or doing something irresponsible. But at some point, the adults couldn't deny the problem with all the cars, purses and possessions still being in the home. Because Sherrill was also missing, I think she was catching a lot of blame from the other adults, just because. She still is, wrongfully so, IMO. The adults may have also been confused because of the stories the kids were telling.
Whatever the excuses given, it doesn't make sense that the McCalls weren't contacted sooner or weren't given enough information to make them head over to the Delmar house immediately. IIRC, she went to the house in the evening and didn't call police until around 10:30 pm.
JMO, its the behavior of the young folks on that day and the weeks after that's problematic. Stories were vague, altered, had gaps in the timeline. I'm guessing there was some drug activity the previous night and early morning; that Stacy and Suzie may not have been at the same parties all evening and that the kids may have been hanging out with other people whose identities they didn't want to reveal - secret boyfriends, secret friends, etc.
Everything sounds odd because some of the details as the public knows them probably aren't accurate. The people who do know the facts are missing.