Sorting loop by 'orderby' 'modified' not working?

Question

I'm trying to build a photostream that sorts the loop by the modified date of a post type. The goal being that as a post is updated with new attachments, it gets pushed to the front of the loop. My problem is that the loop is not being ordered properly. Am I doing something wrong?

$photostream = new WP_Query( array(
    'post_type' => 'inventory',
    'orderby' => 'modified',
    'paged' => $paged,  ) );

if( $photostream->have_posts() ) { 
while( $photostream->have_posts() ){
$photostream->the_post(); 

    the_title('<span>', '</span>, ');

    $mediaargs = array( //check to see if has media
        'order'          => 'ASC',      'post_type'      => 'attachment',
        'post_parent'    => $post->ID,  'post_mime_type' => 'image',
        'post_status'    => null,       'numberposts'    => -1, );
    $attachments = get_posts($mediaargs);
        if ($attachments) { 
            echo do_shortcode('[gallery size="small" columns="8" link="file"]');
            echo '<div class="clear"></div>'; 
        }

} } wp_reset_query();