You've obviously got a ton more experience than I do, but if everything is technically sound and it's still not working, I'd go for something else. No sense in banging your head against a wall. What's your weight as compared to his?
Noob problem i'm having - theres a 76kg Maori fellow, completely shredded and strong as an ox who has a neck i can't bend. Whenever i get the plumb i can't do anything with it even if i get perfect leverage with the step and use all my strength. What do you do with these guys? Just ignore going for the plumb and work head/arm control to score with knees/throws?