Primefaces DataExport always return a empty files

Question

I have a dinamically primefaces dataTable, with 2 columns and some rows. The table by default, always start empty, and the data is loaded with a commandButton. The problem is that the pdf file and the excel file always is returned empty.

The glassfish or the console doesnt show any error.

<h:form id="inputForm" enctype="multipart/form-data">          
<p:panel id="panel" style=""> 

   <p:panel header="Export Page Data">  
       <h:commandLink>  
           <p:graphicImage value="/resources/imagenes/excel.png" />  
           <p:dataExporter type="xls" target="censoTabla" fileName="Censos" pageOnly="true"/>  
       </h:commandLink>  

       <h:commandLink>  
           <p:graphicImage value="/resources/imagenes/pdf.png" />  
           <p:dataExporter type="pdf" target="censoTabla" fileName="Censos" pageOnly="true"/>  
       </h:commandLink>   

</p:panel> 

       <p:selectOneMenu value="#{healthViewBean.aniosValue}" effect="fade" >  
            <f:selectItem itemLabel="Select" itemValue="#{healthViewBean.yearsValue}" />  
            <f:selectItems value="#{healthViewBean.list}" var="task" itemLabel="#{task.label}" itemValue="#{task.value}" />  
       </p:selectOneMenu> 
       <br />
       <p:commandButton update="panel,tabla" value="Load data" actionListener ="#{healthBean.loadData()}" />

</p:panel>

<p:panel id="tabla">
     <p:dataTable id="censoTabla" var="health" value="#{healthViewBean.centros}">  
            <p:column headerText="health status">  
                  <h:outputText value="#{health.centroSalud}" />  
             </p:column>  
             <p:column headerText="No.">  
                   <h:outputText value="#{health.numMedicos}" />  
              </p:column>  
      </p:dataTable>

 </p:panel>
 </h:form>