JSF2.0 popup show after the second click - ajax

Question

I have 3 managebean one model and 2 controllers (1 for the popup text) i want a popup after i insert into the database.
my page is like this the problem is that when i call for mensaje.mostrar() i works but when i try to call cursoControlador.registrarCurso() and from there try to call mensaje.mostrar() that changes the value from mostrarMensaje (boolean) don't show the popup.
<ui:define name="centro">
<h:form>
<h:panelGrid columns="3" cellpadding="5">
<p:outputLabel value="Nombre" />
<p:inputText value="#{cursoModelo.nombre}"/>
this is the button use and works fine
<p:commandButton action="#{mesaje.mostrar()}" value="Registrar Curso" >
<f:ajax render="#form"/>
</p:commandButton>
and this is the behavior i want
<p:commandButton action="#{cursoControlador.registrarCurso()}" value="Registrar Curso" >
<f:ajax render="#form"/>
</p:commandButton>
and this is the popup message i want to show
</h:panelGrid>
<h:panelGroup layout="block" styleClass="fondo-popup" rendered="#{mensaje.mostrarMensaje}">
<div class="panel-popup">
<p:outputLabel value="#{mensaje.texto}"/>
<h:commandButton value="Aceptar" action="#{mensaje.esconder()}">
<f:ajax render="#form"/>
</h:commandButton>
</div>
</h:panelGroup>
</h:form>
</ui:define>
model
#Named(value = "cursoModelo")
#SessionScoped
public class CursoModelo implements Serializable {
private String nombre;
//getters and setters...
}
controller 1
#Named(value = "cursoControlador")
#SessionScoped
public class CursoControlador implements Serializable{
#Inject private CursoModelo cursoModelo;
#Inject private Mensaje mensaje;
/**
* Creates a new instance of CursoControlador
*/
public CursoControlador() {
}
public String registrarCurso(){
//SOME CODE that inserts into de database
mensaje.mostrar(); //this is the problem probably i'm doing it wrong
return "index";
}
}
controller 2 (The controller for the message)
#Named(value = "mensaje")
#SessionScoped
public class Mensaje implements Serializable {
private String texto;
private boolean mostrarMensaje;
/**
* Creates a new instance of Mensaje
*/
public Mensaje() {
texto = "Mensaje a mostrar";
mostrarMensaje = false;
}
public void mostrar(){
mostrarMensaje = true;
}
public void esconder(){
mostrarMensaje = false;
}
//getters and setters from texto and mostrarMensaje
}