Scripts not loaded on node/add pages

I've got a link in my header, the path of which is 'user/login'. I have checked the option in the admin screen to 'Enable for login links' so that clicking the link will load the login form in a colorbox overlay.

This works great on everypage, bar one. The only thing that is different about this page is that it is a node/add page, which I have set to render using my theme, rather than the normal admin screens, via the following addition to template.php:

/**
* Implementation of hook_theme() to enable
* modification of contact us form through template file
* more info: http://drupal.org/node/1092122
* and: http://drupal.org/node/1200216
*/
function transversal_theme($existing, $type, $theme, $path) {
  return array(
    'contact_us_node_form' => array(
      'render element' => 'form',
      // set name of template file
      'template' => 'contact-us-node-form',
      // set path to new template file
      'path' => drupal_get_path('theme', 'theme_name') . '/templates/forms',
    )
  );
} // end function transversal_theme;

For some reason, none of the colorbox scripts are loaded at all on this page. The knock on effect of this is that no colorbox functionality can be used on this page at all - not just the login form pop up.

Do you have any idea why this is? Does colorbox only work on 'front end' (as in viewing, not adding or editing) content? Do you have any suggestions for a work around?

Thanks!