Sub Pages Disappearing

On a client site, I am having trouble where on some pages the sub pages aren’t working at all. My child page listing code is the following:

<div id="sub-nav">
		<?php
		if($post->post_parent)
		$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0"); else
		$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
		if ($children) { ?>
		<h2>Further Reading</h2>
		<ul>
		<?php echo $children; ?>
		</ul>
		<?php } ?>
	</div>

Like stated this is only on certain pages, and it seems to be only pages with a decently sized child listing.

What are somethings I should look into? I’ve tried just running the default wordpress theme and the same issue comes up. It was working fine one day, then blamo not working anymore.