Dynamic SELECT with error on second load

<script src="jquery-1.3.2.min.js" type="text/javascript"></script>
     <!-- DATA PROVA -->
    <script type="text/javascript">
        $(document).ready(function(){
            $().ajaxStart(function() { 
                    $('.carregando').show(); 
                    $('#dataprova').hide();
            });

            $().ajaxStop(function() { 
                $('.carregando').hide(); 
                $('#dataprova').show();
            });

            $('#laboratorio').change(function(){                                    
                $('#dataprova').load('subcategorias.php?laboratorio='+$('#laboratorio').val() );
            });
        });
    </script>
    <!-- HORA PROVA -->
    <script type="text/javascript">
        $(document).ready(function(){
            $().ajaxStart(function() { 
                    $('.carregando').show(); 
                    $('#horaprova').hide();
            });

            $().ajaxStop(function() { 
                $('.carregando').hide(); 
                $('#horaprova').show();
            });

            $('#dataprova').change(function(){                                  
                $('#horaprova').load('subcategorias.php?dataprova='+$('#dataprova').val() );
            });
        });
    </script>

    <style type="text/css">
        .carregando{
            color:#666;
            display:none;
        }

        #carrregando_tipo_2 {
          float: right;
          font-size: 10px;
          margin-right: 10px;
          display:none;
        }       
    </style>


<div class="internas"><div class="titulo"><span>

    <h3>TESTE COMBOBOX</h3></div>
    <div class="dicas">
  <p><select name="laboratorio" id="laboratorio">
    <option>Selecione um tipo</option>

        </select>
</div>
    <!-- DATA PROVA -->
    <div id="carrregando"></div> 

    <span class="carregando"> 
        <img src="loading-02.gif"> Aguarde, carregando...
    </span>

    <span name='dataprova' id="dataprova">

    </span>

    <!-- HORARIO PROVA -->
    <div id="carrregando"></div> 

    <span class="carregando"> 
        <img src="loading-02.gif"> Aguarde, carregando...
    </span>

    <span name='horaprova' id="horaprova">

    </span>

</div>

Only data prova appears. hora prova does not load.