I don't have that problem with my program - a very small group and very little they can do to help out means nothing is ever left un-done and there's really not enough for everyone to do.
So, instead, let me speak to how I dealt with it at my instructor's school. First, by the time it gets to be an issue, the person has been training a couple of years. We really didn't expect anyone below blue belt (usually takes 18 months or so) to even catch on and start helping. Once they've been around that long, the folks working with them (instructors, senior students) tend to know their habits and how they think. We also know details of their lives, from what they share while we're all waiting for class. So, if someone has a busy schedule (busier than normal, having just enough time to make classes), an injury, or something else we're aware of, we just understand that's why they aren't doing it, and it's no big deal. If we know someone has an attitude in other ways, and we see them not helping (and we're not aware of other circumstances), one of the senior students would walk by with a broom or duster and just say something like, "Hey, can you grab a rag and get those mirrors before class?" That was usually all it took. Even folks with an attitude would generally pitch in. Those that didn't, usually just left the school as soon as they saw the students (not the instructors) had a communal expectation that everyone pitched in. They didn't fit in, and they didn't like that, so they went somewhere else. I always hoped they found someplace they fit in better.
In the rare case where someone wasn't pitching in and we had no idea why (no attitude problem, and were there in plenty of time), someone (usually the senior brown belt) would just ask. "Hey, I noticed you haven't been pitching in with the dusting and stuff, man. Is everything okay?" I can only think of two times this happened (in both cases, I was the senior brown belt). Both guys were just preoccupied with some crap going on in their lives. I got them an energetic training partner for the next few classes to work off the stress and offered my ear and a beer if they needed to talk. Eventually, when things got sorted in their lives, they were back to helping out.