Ajax does not work, selectOneMenu in cascade

I'm developing a program but ajax does not work at all. I decided to make a very simple system to see where the error is and even then I can not find a solution. I saw some errors of this in the forum but I could not. HTML          

        <h:outputText value="Pais: " />
        <p:selectOneMenu id="comboPais" process="@this" converter="conversorObjetoId" value="#{comboMB.pais}">
            <f:selectItem itemLabel="--Selecione--" itemValue="" />
            <f:selectItems value="#{comboMB.paises}" var="pais" itemLabel="#{pais.nome}" itemValue="#{pais}" />
            <p:ajax event="change" update="comboEstados" actionListener="#{comboMB.buscar()}" />            </p:selectOneMenu>

        <h:outputText value="Estados: " />
            <p:selectOneMenu id="comboEstados" value="#{vendasMB.estado}">
                    <f:selectItem itemLabel="--Selecione--" itemValue="" />
                    <f:selectItems value="#{vendasMB.estados}" var="estado"  itemLabel="#{estado.nome}" itemValue="#{estado}" />    
                </p:selectOneMenu>      </h:panelGrid>      </p:panel> </h:form>

The index I changed several things already left onChange, ja took update, listener several attempts

@ViewScoped @ManagedBean

@PostConstruct
public void init() {
    paises = dao.consultarPais();
    System.out.println("--> init*()");
}

public void buscar(ActionEvent event) {
    estados = dao.consultarEstados(pais);
}

UPDATED

    <h:form id="frmTeste">
<p:outputPanel id="cadastroCidades">
        <h:panelGrid id="painel" columns="2">

            <p:outputLabel for="comboPaises" value="Pais: " />
            <p:selectOneMenu id="comboPaises" converter="conversorObjetoId"  value="#{comboMB.pais}">
                <f:selectItem itemLabel="--Selecione--" itemValue="" />
                <f:selectItems value="#{comboMB.paises}" var="pais" itemLabel="#{pais.nome}" itemValue="#{pais}" />
                <p:ajax listener="#{comboMB.buscar}" update="comboEstados" />
            </p:selectOneMenu>

            <p:outputLabel for="comboEstados"   value="Estados: " />
                <p:selectOneMenu id="comboEstados" converter="conversorObjetoId"  value="#{vendasMB.estado}">
                        <f:selectItem itemLabel="--Selecione--" itemValue="" />
                        <f:selectItems value="#{vendasMB.estados}" var="estado"  itemLabel="#{estado.nome}" itemValue="#{estado}" />    
                    </p:selectOneMenu>
        </h:panelGrid>
        </p:outputPanel>
</h:form>