Digital BW, The Print

I too am struggling with inconsistencies with posterization on mac 1280 
mis vm paul's curves. some images are beautiful, others seem hopeless. 
But i noticed something interesting today - working on a portrait with 
smooth transition in the skin tones, print was hopelessly posterized. i 
did most of the work in 16bit, histogram was good (no gaps). dodged and 
burned some scratches in 8 bit - histogram good. flattened image  - 
still good - but when i swithched to RGB - gaps in the shadows. I'm 
sure this must be key but i don't know what to do about it. 
suggestions?
tia
suzanne