jQuery table sorter is not working

Code:
echo '<table id="usertable" class="tablesorter">';

      echo '<thead>';

       echo '<tr><th>Felhasználónév</th>';

       echo '<th>Név</th>';

       echo '<th class="centerAlign">Szerkesztés</th>';

       echo '<th class="centerAlign">Törlés</th></tr>';

      echo '</thead>';

      echo '<tbody>';

      

      $i = 1;

      foreach ($all_user->result() as $row) {

          

       $class = ($i % 2 == 0) ? "userManageEvenRow" : "userManageOddRow";

       

       echo '<tr class="' . $class . '">';

          echo '<td class="leftManageUser">';

           echo anchor('user/profile/' . $row->user_name, $row->user_name, array('title' => $row->user_name . ' adatlapja'));

          echo '</td>';

          echo '<td class="centerManageUser">' . $row->surname . ' ' . $row->firstname . '</td>';

          echo '<td class="rightManageUser">';

           echo anchor('admin/edit_user/' . $row->ID, '<img src="' . base_url() . 'images/edit.png" border="0">',

               array('title' => $row->user_name . ' adatlapjának szerkesztése'));

          echo '</td>';

          echo '<td class="rightManageUser">';

           if ($row->ID != $this->session->userdata('ID')) {

            echo anchor('admin/delete_user/' . $row->ID, '<img src="' . base_url() . 'images/delete.png" border="0">',

            array('title' => $row->user_name . ' fiókjának törlése'));

           }

          echo '</td>';

          echo '</tr>';

          $i++;

          

      }

      

      echo '</tbody>';

      echo '</table><br />';