Primefacesダイアログに検証エラーが表示されない

Primefacesダイアログがあり、送信の前後に検証が必要です。ただし、必要な属性がコードで機能しない理由がわかりません。フォームログイン内に同じ必須属性があり、正常に動作します。

<ui:composition>
    <h:body>
        <h:form styleClass="aui">

            <p:messages />
            <fieldset>
                <!-- Start Username -->
                <div class="field-group">
                    <label for="df-username">#{msg['message.user.username.label']}<span class="aui-icon icon-required"> required</span></label>
                    <h:inputText type="text" styleClass="text" name="username" id="df-username" value="#{userBean.username}" required="True">
                        <f:validateLength minimum="4" maximum="30" />
                    </h:inputText>
                    <div class="description">#{msg['message.user.username.desc']}</div>
                    <p:messages for="df-username"/>
                </div>
                <!-- End Username -->
                <!-- Start Password -->
                <div class="field-group">
                    <label for="df-password">#{msg['message.user.password.label']}<span class="aui-icon icon-required"> required</span></label>
                    <h:inputSecret styleClass="text" name="password" id="df-password" value="#{userBean.password}" required="True">
                        <f:validateLength minimum="4" maximum="25" />
                    </h:inputSecret>
                    <div class="description">#{msg['message.user.password.desc']}</div>
                    <p:messages for="df-password"/>
                </div>
                <!-- End Password -->
                <!-- Start Enabled -->
                <div class="field-group">
                    <label for="df-enabled">#{msg['message.user.enabled.label']}<span class="aui-icon icon-required"> required</span></label>
                    <p:selectBooleanCheckbox styleClass="checkbox bk-checkbox" id="df-enabled" value="#{userBean.enabled}" />
                    <div class="description">#{msg['message.user.enabled.desc']}</div>
                    <p:messages for="df-enabled"/>
                </div>
                <!-- End Enabled -->       
            </fieldset>

            <div class="bk-dialog-footer">
                <p:commandButton styleClass="aui-button aui-button-primary bk-aui-button-form" image="aui-icon aui-icon-small aui-iconfont-success" oncomplete="handleDialogClose(xhr, status, args)" value="#{msg['message.save']}" actionListener="#{userBean.save}" /> 
                <h:commandLink type="button" class="aui-button" onclick="PF('dlg-add-user').hide();return false;"><span class="aui-icon aui-icon-small aui-iconfont-close-dialog"/>  #{msg['message.cancel']}</h:commandLink>
            </div>

        </h:form>
    </h:body>
</ui:composition>

なにか提案を？