‘includes’ not retrieving files

Can anyone explain why this might not be working? The files being included contain lists of regions for each country concerned. The first one works, returning a list of counties in England on the page in question. The second one for the USA does not return the expected list of states. The code below is in a file called taxonomy-location.php.

<?php if (is_tax( 'location', 'england' )) { ?>	 	

	 	<?php include('includes/df-location-england.php'); ?>

	<?php } elseif (is_tax( 'location', 'usa' )) { ?>

	 	<?php include('includes/df-location-usa.php'); ?>

	<?php } elseif (is_tax( 'location', 'wales' )) { ?>

	 	<?php include('includes/df-location-wales.php'); ?>

	<?php } elseif (is_tax( 'location', 'scotland' )) { ?>

	 	<?php include('includes/df-location-scotland.php'); ?>

	<?php } elseif (is_tax( 'location', 'northern-ireland' )) { ?>

	 	<?php include('includes/df-location-northern-ireland.php'); ?>

	 <?php } ?>

As an example, here is the code in df-location-england.php:

<?php
		//list terms in a given taxonomy using wp_list_categories (also useful as a widget if using a PHP Code plugin)

		$taxonomy     = 'location';
		$orderby      = 'name';
		$show_count   = 0;      // 1 for yes, 0 for no
		$pad_counts   = 0;      // 1 for yes, 0 for no
		$hierarchical = 1;      // 1 for yes, 0 for no
		$title        = '';
		$child_of     = '167';

		$args = array(
		  'taxonomy'     => $taxonomy,
		  'orderby'      => $orderby,
		  'show_count'   => $show_count,
		  'pad_counts'   => $pad_counts,
		  'hierarchical' => $hierarchical,
		  'title_li'     => $title,
		  'child_of'     => $child_of
		);
		?>

		<ul class="location">
		<?php wp_list_categories( $args ); ?>
		</ul>

The only difference in the USA version of this file is that the child_of ID changes to match the USA, thereby excluding the parent ID from the list.

Any advice appeciated.