Loosing dita tags during ant conversion

At this company, we use a script to convert our .xml files created in FrameMaker into .html files for use in RoboHelp to generate help.

But I am loosing formatting only in my reference files. We use definitions lists (<dl>) extensively in the reference files but the heading tag (<dlhead><dthd>) is being lost and the heading is displaying just like any other definition tag (<dt>). As shown here:

Before running ant conversion (.xml):

<dl>

    <dlhead><dthd>Interval section</dthd></dlhead>

    <dlentry><dt>Interval Type</dt>

    <dd>Select the interval type: Repetitive,

    Not Repetitive, or On Request.</dd></dlentry>

After running ant conversion (.html):

<dl>

<dt><strong>Interval section</strong></dt>

<dt class="dlterm">Interval Type</dt>

<dd>Select the interval type: Repetitive,

Not Repetitive, or On Request.</dd>

Since this is immediately after running ant, it can't be a stylesheet problem, but after two hours of diagnostics, I don't know where to look. I am a writer, not a techie.

Any help is appreciated.