Also, if the stude wears glasses, its worth establishing if they are varifocals or not. Cos to get someone to learn the flare with them on, 8 times out of ten the will not crack it. Get them to use bi-focals to learn the art first.
One thing I have learnt is it can often be the most simplist of reasons why they can't crack it but it needs lateral thinking from the instructor to find it. Explore every concept in you mind whilst they are flying and see if you can elliminate them one by one.
Thats why being instructors is so rewarding.........Is it not??