Hello I have created a custom form added a # ajax callback on form submit

    $form['hotel_book_form']['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Check Availability'),
    '#ajax' => array(
      'callback' => "list_rooms",
      'wrapper' => "replace_hotelbook_div",
      'method' => 'replace',
      'effect' => 'fade',
    ),
  );
and I have added this wrapper as suffix to the form

    $form['hotel_book_form'] = array(
    '#type' => 'fieldset',
    '#title' => t('Book Hotel'),
    '#weight' => 5,
    '#collapsible' => TRUE,
    '#collapsed' => FALSE,
    '#prefix' => '<div id="replace_hotelbook_div"></div>',
  );

but when I click on form's submit button the page gets simply reloaded and the ajax callback function does not fire.