Using Word2003. Not a Word expert.

I'm preparing a series of bifold, duplex booklets that share a lot of
common pages. The full 'master' is 40 pages, and there are 4 page, 8
page, 16 page, 20 page, 28 page variations that are a subset of the 40
page master.

This seems like a good usage of IncludeText, and it "sort of" works.
Below is an example of what I'm trying to accomplish.

Page01.doc thru Page40.doc

IncluderOfFourPages.doc has:
{ INCLUDETEXT "Page01.doc" }
{ INCLUDETEXT "Page11.doc" }
{ INCLUDETEXT "Page21.doc" }
{ INCLUDETEXT "Page31.doc" }

The problem is when I "refresh" the "master" with Ctrl-A and F9, I'm
getting
Error! Not a valid filename
for some or all of the references.

It seems to work at least once when I build up the subpages initially.
Eventually, I start getting the Error! message as if it is corrupted.

Am I doing something wrong? Leaving out a step? Is there something
with the booket that confuses Word-2003? It is quite close to working,
but frustrating.

A year or so ago, I was working on a similar project, and it seems
like there was a "magic step" involved to get the "refresh" to work ok
.... but I can't remember what it was. I would encounter the above
"Error! Not a valid filename" and it seemed there was something I
could do to fix it so the "refresh" would work ok. Can't recall what
it was. ???