current_page_item not displaying using wp_list_pages

Hello,

I have used this code here:

<?php
if(!$post->post_parent){
	// will display the subpages of this top level page
	$children = wp_list_pages("title_li=&child_of=".$post->ID."&echo=0");
}else{
	// diplays only the subpages of parent level
	//$children = wp_list_pages("title_li=&child_of=".$post->post_parent."&echo=0");

	if($post->ancestors)
	{
		// now you can get the the top ID of this page
		// wp is putting the ids DESC, thats why the top level ID is the last one
		$ancestors = end($post->ancestors);
		$children = wp_list_pages("title_li=&child_of=".$ancestors."&echo=0");
		// you will always get the whole subpages list
	}
}

if ($children) { ?>
<div id="sub-pages-sidebar">
	<ul>
		<?php echo $children; ?>
	</ul>
</div>
<?php } ?>

to display the subpage of a page on the sidebar, on this website I did before is displays all fine with a selected class http://www.colchesteryogastudio.co.uk/yoga-classes/parent-baby-yoga/
but, on this other website I have done
http://new.simplydesign.co.uk/services/web-development/
the current_page_item class is not displaying.

Can anyone help please? Thank you