Vocations as the crowning glory of Youth Ministry

I've heard this phrase often... 'Vocations as the crowning glory of Youth Ministry' and feel guilty every time I hear it. I'm the only one to survive from Don Bosco school, Sanathnagar!! Among the thousands who've passed out of Don Bosco, Sanathnagar why is it that only one opted and is still a Salesian. In the Vietnam and Thailand provinces, I'm told that there get a good number of vocations from the tribal groups we work with. Why is this not true of the dozen technical and boarding schools that we have in the Province? After so many years of living, interacting and being enriched by us, why don't they make that final 'yes' to become Salesians? Something lacking somewhere...