I've written a module which loads CSS and JS files using the #attached property.  This works fine across most themes, but for some themes the files are not loading.

e.g.:

    $page['header']['#attached']['css'][] = array(
      'data' => drupal_get_path('module', 'mymodule') . '/css/mymodule_default.css',
      'type' => 'file',
      'options' => array(
        'group' => CSS_THEME,
        'preprocess' => TRUE,
        'every_page' => TRUE, 
      ),
    );
    
As there is very little by way of explanation on #attached (most articles repeat the same stuff) I can't figure out where the problem may lie.  Using `drupal_add_css()` and `drupal_add_js()` works, but is not recommended for caching reasons, and I prefer to pass settings using attached for this reason.

The #attached properties are showing in the `$page` array but are not being rendered.

In summary, are there situations when certain themes block the correct rendering of #attached elements that would normally be applied to a render structure by drupal_process_attached()

Any help is appreciated.