problem with the triangle?

I have a student in my class who is 6'2 and weighs about 145 lbs.he has long thin legs and when he tried applying the triangle choke on another student last night he was applying the choke in textbook fashion but for some reason it wasn't having an effect on his opponent.The guy he was grappling was 5'8/125 if that accounts for anything.

Is there something special he needs to do to modify the choke due to his lankiness?Is the problem not really with him but with the fact that he was going against a much smaller opponent?What can he do to correct this problem?Thanks,Miller

Squeeze his knees together, THEN pull on the head. Always works.

is he raising his pelvis as he pulls the head down?