minor grouping issue

I've got a book, where I [expletive deleted] in the individual chaptes and hold them
in a variable.  I want to export the footnotes as a separate file,
wherein the notes are grouped and numbered by chapter.

If I use the below, however, I end up with a group for each chapter,
but each group contains all the notes for the entire book.

<xsl:for-each-group select="$chapters" group-by="db:chapter">
     <h3>Chapter <xsl:value-of select="current-group()/@label"/></h3>
      <xsl:apply-templates select="current-group()//db:footnote"
mode="footnote-list"/>
</xsl:for-each-group>

What am I doing wrong?

Bruce