I have a centrifugal one (not the absolute best for juice quality, but generally very good for quantity).
Green leaves can be tricky! If I push them in the wrong way they just end up being pushed aside...I have to put them in so they get shredded and juiced...the blade has to catch the leaves across the edges rather than along the face if you follow me...
Cabbage I cut into wedges before juicing, and spinach I roll up several leaves and feed in long ways. Hope that makes sense!
With one of the "better" juicers there may be a different way of maximising your output.