We all have off nights. And sometimes there's no explanation. If it's just one or two don't worry. If it's a trend then try to figure out what's happening.
As a host you can tell if songs with expletives are going over or not just by looking over the room. My place accepts those songs... even embraces them sometimes. And we're pretty steadily busy. But our concentration is on recent material (because no one else in this area was doing that basically) and it's not for all rooms. For sure.
If there's a sudden drop-off in business it's usually because of something that just happened. A price hike or new cover are the usual suspects, but I'd put a police raid
in that category. If the anything-goes language had been there all along, that wouldn't be where I'd look first.
As Karen said, communication with your group in all things is a pretty good way to gauge where they are.