problem with xrefs - maybe FOP 0.20.5 issue?

[Marshall Schor <msa@schor.com>]

I'm using Docbook 4.5 / 1.71.1 with FOP 0.20.5.

I use cross references with olinks.  They work most of the time, but 
occasionally it seems that the output for page number references to 
cross refs in the same book (I'm using modular files - that's why the 
olinks are there even for the same book) show as [?] and are not active. 

I looked at the XSL:FO input to FOP - it all looks like:

  <fo:basic-link color="blue" internal-destination="ugr.tug.application">
    Chapter 3, Application Developer's Guide
      <fo:basic-link color="blue" 
internal-destination="ugr.tug.application">
        <fo:inline keep-together.within-line="always">
           [<fo:page-number-citation ref-id="ugr.tug.application"/>]
        </fo:inline>
      </fo:basic-link>
  </fo:basic-link>


Sometimes this seems to work, other times it seems to fail (showing a 
"?" for the page number, and/or not making the link "active" in the 
resultant PDF.  (The target id "ugr.tug.application" is present, and is 
referred to successfully from the TOC).

There are no messages coming from FOP for this (associated with these 
pages).  And these same kinds of refs in other places in the source 
input seem to work just fine.  I haven't been able to see a pattern to 
the failure.

I would appreciate any advice on how to work-around this issue.

-Marshall Schor