Digital BW, The Print

The top-feeder of 2200 seems under-designed...I suspect it wants
papers of a certain flexibility, and not of other flexibility even
though they may be of the same thickness. Just a hypothesis. As
possible evidence, I've noticed that some two-sided papers feed easily
on the first pass, but not at all on the second pass. 

Some (including me) have suspected that the mechanism wears quickly
with heavy papers, causing troublesome feed from the top.

Looking at the top feed mechanism and feed throat from the top, as it
feeds papers, I wonder if a raised fin or bump in the middle, that
would cause the paper to feed with the middle of the paper raised a
little, might help? 

Has anybody done any modifications like this? 

Do you have or know of a diagram of the feed mechanism, one that would
show all parts as well as paper flow?