Two bindings; One Works, Other Doesn't. Why? - wpf

Question

Why does one label update but not the other? Both bound to the same property. I assume there's a problem with the binding being in a dataTemplate? Using Resharper I'm told that my lblOverallInt cannot resolve the symbol. How could I fix this?
<Label Name="lbl1" Content="{Binding Path=lblOverallInt, UpdateSourceTrigger=PropertyChanged}"/>
<Expander>
<Expander.HeaderTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Label Name="lbl2" Content="{Binding Path=lblOverallInt, UpdateSourceTrigger=PropertyChanged}"/>
</StackPanel>
</DataTemplate>
</Expander.HeaderTemplate>
</Expander>
UPDATE
Related follow on question; I have the following style binding also which works when applied to the label but not to the Expander. Is there a similar process for wiring this up as mm8 solution to the top part of this question?
Added separate solution for this part
<Expander.Style>
<Style TargetType="{x:Type Expander}">
<Style.Triggers>
<DataTrigger Binding="{Binding Path=lblOverallInt}" Value="0">
<Setter Property="Foreground" Value="Yellow"/>
</DataTrigger>
<DataTrigger Binding="{Binding Path=lblOverallInt, Converter={StaticResource isZeroConverter}}" Value="False">
<Setter Property="Foreground" Value="Red"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Expander.Style>

Answer 1

The DataContext of the HeaderTemplate is the header itself. Try this:
<Expander>
<Expander.HeaderTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Label Name="lbl2" Content="{Binding Path=DataContext.lblOverallInt, RelativeSource={RelativeSource AncestorType=Expander}}"/>
</StackPanel>
</DataTemplate>
</Expander.HeaderTemplate>
</Expander>
Or this:
<Expander Header="{Binding Path=lblOverallInt}">
<Expander.HeaderTemplate>
<DataTemplate>
<StackPanel Orientation="Horizontal">
<Label Name="lbl2" Content="{Binding}"/>
</StackPanel>
</DataTemplate>
</Expander.HeaderTemplate>
</Expander>

Answer 2

For part 2 of my question I had the style binding part still attached to the Expander when it actually needed to be included in the DataTemplate;
<Expander.HeaderTemplate>
<DataTemplate>
<Border Height="24">
<StackPanel Orientation="Horizontal">
<StackPanel.Style>
<Style TargetType="{x:Type StackPanel}">
<Style.Resources>
<Style TargetType="{x:Type Label}">
<Style.Triggers>
<DataTrigger Binding="{Binding}" Value="0">
<Setter Property="Foreground" Value="Yellow"/>
</DataTrigger>
<DataTrigger Binding="{Binding Converter={StaticResource isZeroConverter}}" Value="False">
<Setter Property="Foreground" Value="Red"/>
</DataTrigger>
</Style.Triggers>
</Style>
</Style.Resources>
</Style>
</StackPanel.Style>
<Label>Errors/Warnings:</Label>
<Label Name="lbl2" Content="{Binding}"/>
</StackPanel>
</Border>
</DataTemplate>
</Expander.HeaderTemplate>