AJAX doesn't work repeatedly in Drupal 7

I've faced with an AJAX issue in Drupal 7.14

I have the next topic.
It is necessary to have a sum result (form i.e. 5 textfields) when somebody push Submit. It works. But when I change some data in the fields and push again the result remains the same as it was the first time.

I guess that AJAX doesn't reload array $form_state, when ajax_calculate_result is callbacked

Have you got any thoughts where the root of issue is?

Hope your help.
The actual code is below:

function some_module_calculate_form($form, &$form_state, $tid) {

  foreach ($tid as $service_type) {    
    $form['reckon_'.$service_type->aid] = array(      
      '#type' => 'textfield',
      '#title' => t('service measure pattern'),
      '#size' => 5,
      '#maxlength' => 5,
    );  
  }

  $form['submit'] = array(
    '#type' => 'submit', 
    '#value' => t('Calculate'),
    '#ajax' => array(
      'callback' => 'ajax_calculate_result',
      'wrapper' => 'calculate-rezult',
      'effect' => 'fade',
      'method' => 'replace',
    )
  ); 
  $form['result'] = array(
    '#markup' => 'Some result will be here',
    '#prefix' => '<div id="calculate-rezult">',    
    '#suffix' => '</div>',
  );  
  return $form;
}


function ajax_calculate_result($form, &$form_state) {
  
  $sum = 0;
  
  foreach ($form_state['values'] as $k=>$v) {        
    $sum += $v; 
  }
  
  $form['result'] = array(
    '#markup' => 'Total sum is ' . $sum,
  );  

  return $form['result'];
}