DataGridTemplateColumn ComboBox in CellTemplate interfering with binding update

The block of code below is to allow the user to select a line style for a series. The combobox will have several drawn line styles to choose from.

If I comment out the CellTemplate section then the property the SelectedIndex is bound to updates fine. With the section in there it seems to completely prevent the property from ever being updated. Any ideas what I am missing?

<DataGridTemplateColumn Header="Line" Width="80" x:Key="LineStyleColumn"
                                Visibility="{Binding DataContext.IsContour, Source={x:Reference EditorControl}}"><DataGridTemplateColumn.CellTemplate><DataTemplate><ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DisplayDashData}"                                  
                              SelectedIndex="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.activeChart.SelectedTire.LineStyleIndex, UpdateSourceTrigger=PropertyChanged}"
                              ItemTemplate="{StaticResource LineStyleTemplate}"
                              Style="{StaticResource seriesCBStyle}" IsReadOnly="True"></ComboBox></DataTemplate></DataGridTemplateColumn.CellTemplate><DataGridTemplateColumn.CellEditingTemplate><DataTemplate><ComboBox ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DisplayDashData}"
                              ItemTemplate="{StaticResource LineStyleTemplate}"
                              SelectedIndex="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Window}}, Path=DataContext.activeChart.SelectedTire.LineStyleIndex, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                              Style="{StaticResource seriesCBStyle}"></i:Interaction.Triggers></ComboBox></DataTemplate></DataGridTemplateColumn.CellEditingTemplate></DataGridTemplateColumn>