Using Ajax-Callback with ajax_command_invoke to invoke custom jQuery command
Using drupal_add_js() in an AJAX callback will (at best) have weird results. Come to think of it I'm not sure I've ever actually seen it work.

The simplest solution is to add the JS in the form function itself, that way it'll always be available regardless of the AJAX state of the page:

function MYMODULE_some_form($form, &amp;$form_state) {
  // Moved from the AJAX callback to here.
  $form['#attached']['js'][drupal_get_path('module', '&lt;...&gt;') . '/js/jquery_plugins.js'] = array();

  // ... Rest of the form
}