My callback function in my form doesn't execute

Hi, first of all thanks for any response. I present my problem:

I created a block with the following code:

/**
 * Implementacion de hook_block_info().
 */
function rc_block_info() {
  return array(
    'rc-form-filter' => array(
      'info' => t('Contenedor de formulario para filtrado de productos.'),      
    ),
  );
}

/**
 * Implementacion de hook_block_view().
 */
function rc_block_view($delta) {
  $block = array();
  
  if($delta == 'rc-form-filter'):
    $block['subject'] = null;
    $block['content'] = render(drupal_get_form('rc_form_filter'));
  endif;
  
  return $block;
}

and the content of the block is this form with their callback function respective (rc_form_filter_callback) and others functions for obtain the options values:

/**
 * Retorna una array de paises.
 * 
 * @param $tid
 *    Es el identificador del continente del que se quiere obtener los paises.
 * @param $vid
 *    Es el identificador del del vocabulario que contiene los continentes y paises.
 * 
 * @return
 *    Un array cuyas llaves son las tid (taxonomy id) del pais y cuyos valores son los nombres de los paises.
 */
function rc_get_countries($vid, $tid) {
  $values = array();
  
  if($tid != 0):
    $terms_objs = taxonomy_get_tree($vid, $tid, 1);
    
    foreach($terms_objs as $term_obj):
      $values[$term_obj->tid] = t($term_obj->name);
    endforeach;
  endif;
  
  return $values; 
}

/**
 * Retorna una array de continentes.
 * 
 * @return
 *    Un array cuyas llaves son las tid (taxonomy id) del continente y cuyos valores son los nombres de los continentes.
 */
function rc_get_continents() {
  $vocabulary  = taxonomy_vocabulary_machine_name_load('continents_countries');
  $terms_objs  = taxonomy_get_tree($vocabulary->vid, 0, 1);
  $values['vid'] = $vocabulary->vid;
  
  foreach ($terms_objs as $term_obj):
    $values['continents'][$term_obj->tid] = t($term_obj->name);
  endforeach;
  
  return $values;
}

/**
 * Callback functuion for ajax responds
 */
function rc_form_filter_callback($form, &$form_state) {
  return $form['rc-countries'];
}

/**
 * Crea un formulario para filtrar por continentes y paises.
 */
function rc_form_filter($form, &$form_state) {
  $vocabulary_options = rc_get_continents();  
  $default_value      = isset($form_state['values']['rc-continents']) ? $form_state['values']['rc-continents'] : 0;
  
  $vocabulary_options['continents'][0] = '';
  
  $form['rc-continents'] = array(
    '#type' => 'select',
    '#prefix' => '<div id="continents">',
    '#title' => t('Continente:'),
    '#id' => 'continents',
    '#options' => $vocabulary_options['continents'],
    '#default_value' => 0,
    '#suffix' => '</div>',
    '#ajax' => array(
      'event' => 'change',
      'callback' => 'rc_form_filter_callback',
      'wrapper' => 'countries-replace',
    ),
  );
  $form['rc-countries'] = array(
    '#type' => 'select',
    '#prefix' => '<div id="countries-replace">',
    '#id' => 'countries',
    '#title' => t('Pa&iacute;s'),
    '#default_value' => '--',
    '#options' => rc_get_countries($vocabulary_options['vid'], $default_value),
    '#suffix' => '</div>',
  );
  
  return $form;
}

The page is displayed either and the block with the form also, but when the first selected box change the second selected box doesn't change, I don't received any error message.

Thank's for any help

Sorry for my english XD.