CSS isn't updated after being changed

When the CSS is changed the file isn't updated, so the changes are never visible. The code that controls loading the CSS file is as follows:

    // Check to see if there is any CSS.
    if (!empty($panelizer->css)) {
      ctools_include('css');
      $filename = ctools_css_retrieve($display->cache_key);
      if (!$filename) {
        $filename = ctools_css_store($display->cache_key, $panelizer->css);
      }
      drupal_add_css($filename, array('group' => CSS_THEME));
    }

Nothing is done when the CSS is changed so once the CSS is loaded the first time it stays that way.