What might cause a featured image not to display?

The php extension PHP GD was not installed on my dev machine. Installing php5-gd resolved this – sudo apt-get install php5-gd

Stepping through using xdebug revealed that the theme’s aq_resize function was failing at the following code:

else {

    $editor = wp_get_image_editor($img_path);

    if ( is_wp_error( $editor ) || is_wp_error( $editor->resize( $width, $height, $crop ) ) )
        return false;

    $resized_file = $editor->save();

    if(!is_wp_error($resized_file)) {
        $resized_rel_path = str_replace( $upload_dir, '', $resized_file['path']);
        $img_url = $upload_url . $resized_rel_path;
    } else {
        return false;
    }

}

$editor was set to ‘No editor could be selected.’