So called “burnout” is not the primary reason that some rabbonim are not effective in chinuch (even if they are otherwise talmedei chachamim and ehrliche yidden). Most yeshivos do not have a robust pre-hiring program to evaluate the educational skills of prospective teachers and in most cases, rely upon referrals to the rosh yeshiva or cursory trial shiruim.
The OP wrote, “I don’t know why it is this way but at some point, even a successful rebbi can lose his ability to be the best teacher of Torah. ”
So you are talking about a totally separate issue.