Mouse down handling not working as expected using ComboBoxItem

I have a "thing" with the Mouse down not handling as expected using a ComboBoxItem: the selected item event is not triggering. Using snoop I can see that the mouse event is sometimes triggering differently on the children uielements of the ComboBoxItem; when the selected item works the event triggers on the comboboxitem border (name = BG); when it does not work the event is triggered on the comboxitem Textblock.

Anybody have an thoughts? Thanks.

I have the comboxitem defined as:

<ComboBox.ItemTemplate><DataTemplate><ComboBoxItem 
   IsEnabled="{Binding Path=IsChild}"
   Visibility="{Binding IsChild, Converter={StaticResource BooleanToVisibilityConverter}}"
   Content="{Binding Converter={StaticResource ScenarioTextConverter},ConverterParameter={x:Static lim:Models.ScenarioDisplayFormat.PrefixedForComboBoxDisplay}}" 
   Background="Transparent"></ComboBoxItem></DataTemplate></ComboBox.ItemTemplate>