if this is a problem on your site check that there isn't a
condition set on $post['signature'] ... you may also have to check the files (if you have hacks installed)
are you asking how to do this? if so try...
Code:
<if condition="($post['signature'] OR $show['sigimage']) AND $post['postcount']!=1">
<!-- sig -->
<hr size="1" style="color:$stylevar[tborder_bgcolor]" />
$sigimageabove
$post[signature]
$sigimagebelow
<!-- / sig -->
</if>