Over 200 students, wow that's quite a bit. For a small school with maybe 10 or 12 students at the most I can see how it wouldn't be that hard to develop close relationships with all your students and to know exactly where they are in terms of skill level but if you've got over 100 students, or for that matter 200 students it must be very difficult unless you've got lots of assistant instructors.
As for a student not progressing in rank for 10 years it could be because of a bad relationship with the instructor, or it could be that maybe the student just doesn't care to promote any further. People take up the martial arts for all different reasons, desires, and goals. For some people rank progression just isn't one of those reasons, desires, or goals.