I've been monitoring this issue throughout the past few FOP releases, and it
doesn't appear to be fixed yet nor can I find a bug report on it.  This works in
0.20.5 but not in 0.94 or even trunk.

The space-after property on blocks inside a table-header do not work. 
Padding-after does.

Modifying the table.fo example file to have a table-header with a space-after
block will easily recreate the issue:

    <fo:table table-layout="fixed" width="100%" border-collapse="separate">
      <fo:table-column column-width="50mm"/>
      <fo:table-column column-width="50mm"/>
      <fo:table-column column-width="50mm"/>
      <fo:table-header>
        <fo:table-row>
          <fo:table-cell number-columns-spanned="3">
            <fo:block border="2px solid black" space-after="12pt"
text-align="center">Table Header</fo:block>
          </fo:table-cell>
        </fo:table-row>
      </fo:table-header>
      <fo:table-body>
        <fo:table-row>
          <fo:table-cell><fo:block>good</fo:block></fo:table-cell>
          <fo:table-cell><fo:block>bad</fo:block></fo:table-cell>
          <fo:table-cell><fo:block>ugly</fo:block></fo:table-cell>
        </fo:table-row>
      </fo:table-body>
    </fo:table>

I will attach output from 0.20.5 and output from trunk.