hook_form_alter() adding ajax event
You could try adding an 'event' item to the ajax array in your FAPI code. The form needs to know when to trigger the ajax callback. The FAPI documentation says you don't have to specify it explicitly but try 'event' => 'change', and see if that works for you. Since you are not specifying it, perhaps the default one that Drupal uses is not right for a select type.  

See https://api.drupal.org/api/drupal/developer%21topics%21forms_api_reference.html/7.x#ajax_event