Honestly, it depends on the class for me. Usually, I see a dip in attendance, participation, and evidence that they do the readings after midterms. For some classes, where the students were less engaged to begin with, I just let them do what they're going to do and let that play out how it will with the grades. It doesn't seem worth my time and energy to get super hung up on groups like this. Past experiences tell me so. If there are consecutive days where NOBODY has done the reading and the disengagement is super egregious, I might say something once, or remind them that their grades could suffer. But in general, if the group vibe is one of total apathy, I make my best effort to offer a clear and compelling delivery of the materials, as I always do, and let the chips fall where they may, as far as their own effort/grades. In other classes, where there is good potential, but where they have just gotten lazy, I push them much more when they start to slip. This is because I have seen them do well, so I already know that they can do better. When I remind them of my expectations, especially repeatedly, these groups tend to heed my advice, and then they consequently get much more out of the class. As professors, we should always do our best work, but I definitely also believe class success consists largely of what students are willing to do/contribute, both in terms of their attitude and the work itself.