You hardly posted enough information to guess an answer, but

  <xsl:if test="following-sibling::TopNode[@name = $constraint]"> 

xsl:if is just a boolean test it doesn't change the current node.
If you want to change the current node to that node (if it is there)
change that to

<xsl:for-each test="following-sibling::TopNode[@name = $constraint]"> 

David

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