groupBy="true" not working

[Steve Torrefranca]

I have been struggling with groupBy="true". The Group by column as shown 
in http://www.irian.at/myfaces/simpleGroupBy.jsf does not work in my code:

<t:dataTable value="#{test.items}" var="item">
<t:column groupBy="true">
<f:facet name="header">
<t:outputText style="font-weight: bold;" value="Col1" />
</f:facet>
<t:outputText value="#{item.colItem1}" />
</t:column>
<t:column>
<f:facet name="header">
<t:outputText style="font-weight: bold;" value="Vendor"/>
</f:facet>
<t:outputText value="#{item.colItem2}" />
</t:column>
</t:dataTable>