I surmised as much... you are sending pages 2-2 to the center branch (from the second page to the second page). It should be 2 - -2 (from the second page to the penultimate page).
Still not working for me,
I have a job with pg 1 as a single, pages 2 -127 as spreads and page 128 as a single.
When I run through the workflow, it just creates a PDF of pages 1,2,3,128
Note This post was moved to a separate thread. It was originally added to this thread: http://forum.support.xerox.com/t5/FreeFlow-Core/Unimposing-booklets-laid-out-in-reader-s-spreads-a-s...