years ago we had a guy who could do backward rolls like nobody business, but for the life of him, he couldn't do forward roll. we tried all kind of approach and nothing work. the forward roll, even at kneeling level, freaked him out. there was an incredible amount of fear that he could not overcome.
maybe try blindfold then let your student do forward roll at sitting level. or try the systema approach https://www.youtube.com/watch?v=bGfuK9H1RKk