If you have one of these students why not humbley approach them and ask them why they left? The answer might be eye opening.
I have... and the one I'm thinking of wanted a good workout, didn't like aerobics, and didn't want to test - so every time the instructor of the class he was in told him he was ready to test and when the testing was, he switched to another class. He had a circuit he followed - there were 5 or 6 instructors on his route - what he didn't realize was that they were all friends, and talked to each other about new students who had come from within the organization, and when he started the second circuit, they figured it out pretty quickly. When he started the third circuit, they asked him to pick one class and stay there - because it was confusing to other students who knew he was training regularly, had the skills, and never tested.
When I asked him about it one night (after hearing him list the 4 or 5 other instructors he'd trained with within the USTF, within about 14 months - he was a 3rd or 4th gup blue belt at the time), he told me that TKD was the only workout he could stick with, but he just wasn't interested in testing... and every time he stayed in a class a few months, the instructor would tell him it was time to test, and he'd leave, find another class, tell the instructor his work hours and/or location had changed as his reason for changing classes.
As an instructor, this type of student is quite frustrating - because as an instructor, I want to make sure every student gets individual attention, but "tourists" like the one above take attention from those who truly want to progress, and then they leave, to the detriment of the other students. This is the type I'm talking about - not those who move for work or family or because they haven't found the class where they really fit.