Generating output from XML and CSS with docbook - working for HTML, failing for PDF

Question

Hello. Starting from a series of xml files and a stylesheet I generate a HTML file using docbook-xsl-ns-1.73, libxslt-1.1.22.win32 and fop-0.94. the stylesheet says that code chunks must have numbered lines and a gray background. here is the piece of the and task that converts xmls to html:

<exec executable="${XSLTPROC_EXECUTABLE}">
     <arg value="--output" />
     <arg value="${target.html.dir}/${book}_toc.xml" />

     <arg value="--xinclude" />

     <arg value="--param" />
     <arg value="html.stylesheet" />
     <arg value="'css/visixd-docbook.css'" />

     <arg value="--param" />         
     <arg value="chunk.separate.lots" />
     <arg value="1" />

     <arg value="--param" />         
     <arg value="chunk.first.sections" />
     <arg value="1" />

     <arg value="--param" />
     <arg value="chunk.section.depth" />
     <arg value="8" />

     <arg value="--param" />         
     <arg value="toc.section.depth" />
     <arg value="3" />

     <arg value="--param" />         
     <arg value="navig.graphics" />
     <arg value="1" />

     <arg value="--param" />         
     <arg value="ignore.image.scaling" />
     <arg value="1" />

     <arg value="src/reusable/xsl/makeCustomToc.xsl" />
     <arg value="src/workspace/${book}/${language}/${book}.xml" />
    </exec> 

now this task works perfectly fine, producing the desired output.

the similar task used for converting xmls to pdf has these parameters:

<exec executable="${XSLTPROC_EXECUTABLE}">
     <arg value="--output" />
     <arg value="${target.fo.dir}/${book}.fo" />

     <arg value="--param" />
     <arg value="paper.type" />
     <arg value="'USletter'" />         

     <arg value="--param" />
     <arg value="img.src.path" />
     <arg value="'src/workspace/${book}/${language}/'" />

     <arg value="--param" />
     <arg value="fop.extensions" />
     <arg value="0" />

     <arg value="--param" />
     <arg value="fop1.extensions" />
     <arg value="1" />

     <arg value="--param" />
     <arg value="pdf.stylesheet" />
     <arg value="'css/visixd-docbook.css'" />

     <arg value="--xinclude" />

     <arg value="--param" />         
     <arg value="chunk.separate.lots" />
     <arg value="1" />

     <arg value="--param" />         
     <arg value="chunk.first.sections" />
     <arg value="1" />

     <arg value="--param" />
     <arg value="chunk.section.depth" />
     <arg value="8" />

     <arg value="--param" />         
     <arg value="toc.section.depth" />
     <arg value="3" />

     <arg value="${DOCBOOK_XSL_HOME}/fo/docbook.xsl" />
     <arg value="src/workspace/${book}/${language}/${book}.xml" />
    </exec>

the problem is my ant task for pdf conversion fails to format code lines as stated in the stylesheet. what can be wrong? considering that the conversion to html uses the same stylesheet and works. where should i look for a solution? thanks in advance.