QTR-Quadtone RIP

My apologies if this has been broached before. I searched the archives
and couldn't find an answer based on my keyword parameters so I throw
this out to the list.

I've been printing for over a year on an R2400 using QTR. Normally I
proof on letter size Epson Enhanced matte (or whatever it's now
called) using the sheet feed. I would then do final prints with Epson
Velvet Fine Art also using sheet feed; usually with a piece of
foamcore behind it for support. I recently found the support for the
manual rear feed and I've had success doing my larger 13x19 prints
using it. But, and here's the rub, when I went to make letter-size VFA
final prints with the rear feed (manual) setting in QTR I find that
the prints aren't centered as they were with my proofs done through
the sheet feed. The border is diminished on one side of the long axis.

I've benefited tremendously from this group and now I place myself
humbly before your assembled knowledge.

Carl Sander Socolow