The way you describe the problem here, they sound uninspired, and maybe bored. I might be off, but consider what they are working on at that point in their training. Does it get more technical and less physical at that point? Is there a sharp drop-off in new stuff to work on? Are they spending too much time working on what the lower belts need, so they're not getting to work on what they need to progress?Our Kids classes used to be a smaller part of our school but over the past 3 years became the biggest part without trying. Our kickboxing class does great too but that is marketed purely for fitness. When I say serious I don't mean live in the dojo type of thing, I just mean that you are giving it your all and actually trying to improve. This usually isn't a problem with beginners, but as they go up in rank they seem to try less. I have about 5 or 6 serious adult students that have been with me any where from 2 to 8 years but we seem to only be able to keep most adults around for 6 months to a year. I often wonder it location is part of it since the majority of work in my town is manual labor, but that wouldn't explain why our kickboxing class does so well. There is a BJJ school in town that I train at a few times a week and even their adult classes only have about 6 on the mat at any given time, while their kids class has about 25 at any given time.
I think what gpseymour said is pretty accurate, Time commitments are hard to shift, and habits hard to alter, so some folks get started then find it's too difficult keeping the schedule they planned.