Found the answer....
<ComboBox x:Name="cboChildAccount" Grid.Column="1" SelectedValue="{Binding Path=accFrom, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="ID" ItemTemplate="{StaticResource AccountTemplate}" Width="120" HorizontalAlignment="Left" VerticalAlignment="Center" IsTextSearchEnabled="True" TextSearch.TextPath="comment" IsSynchronizedWithCurrentItem="False" SelectionChanged="cboChangedC">
<ComboBox.GroupStyle>
<GroupStyle ContainerStyle="{StaticResource CustomExpanderComboGroupItemStyle}" HeaderTemplate="{StaticResource GroupHeader}"/>
</ComboBox.GroupStyle>
<ComboBox.Style>
<Style TargetType="ComboBox" BasedOn="{StaticResource RoundedComboBox}">
<Style.Triggers>
<DataTrigger Binding="{Binding Path=parent, Converter={StaticResource IsNullConverter}}" Value="True">
<Setter Property="ItemsSource" Value="{Binding Source={StaticResource RealData}}"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=parent, Converter={StaticResource IsNullConverter}}" Value="False">
<Setter Property="ItemsSource" Value="{Binding Source={StaticResource VirtualData}}"/>
</DataTrigger>
</Style.Triggers>
</Style>
</ComboBox.Style>
</ComboBox>
and remove the Triggers from the DataTemplate.Triggers
section
Andy