DataGrid : Problem binding Items to Combobox and Properties

In my UserControl, I have 3 DataGrid. In each DataGrid, I have combobox in Header. I am able to bind the list of DataGrid, but not able to bind the lsit of items for Combobox & its Selected property. Below is the code :

<DataGrid AutoGenerateColumns="False" Name="squeezeDg" Height="170" Width="550" BorderBrush="#FFB7B39D" HorizontalAlignment="Left" VerticalAlignment="Top" HorizontalScrollBarVisibility="Visible" VerticalScrollBarVisibility="Visible"        Background="LightYellow" RowBackground="LightGray" AlternatingRowBackground="#FFFFFFF5" BorderThickness="10"  CanUserReorderColumns="False" CanUserSortColumns="False" CanUserDeleteRows="False" CanUserAddRows="False"     FontSize="13" ItemsSource="{Binding StageModelList, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" Margin="0,0,-1,0"  ><DataGrid.Columns><DataGridTextColumn Header="Stage" Width="75" Binding="{Binding StageName, ValidatesOnExceptions=True}" /><DataGridTextColumn Width="60" Binding="{Binding Bhp, Mode=TwoWay, ValidatesOnExceptions=True}"><DataGridTextColumn.Header><Grid Width="50"><Grid.RowDefinitions><RowDefinition Height="25" /><RowDefinition/></Grid.RowDefinitions><TextBlock Text="BHP" Grid.Row="0"/><ComboBox Grid.Row="1" Width="50" HorizontalAlignment="Center" Name="cboBhp" 
                                               ItemsSource="{Binding Path=DataContext.InjItems, RelativeSource={RelativeSource AncestorType=UserControl}}" SelectedValue="{Binding Path=DataContext.BhpUnit, RelativeSource={RelativeSource AncestorType=UserControl}, Mode=TwoWay}"></ComboBox></Grid></DataGridTextColumn.Header></DataGridTextColumn>

Model object who is set as the DataContext for the UserControl i.e. object Step3InfoData

private string bhpUnit;

public Step3InfoData()
{
    init();
    InjItems = new ObservableCollection<string>();
    InjItems.Add("pascal");
    InjItems.Add("Braye");

     BhpUnit = "pascal";
}

public string BhtUnit
{
      get { return bhpUnit; }
      set
      {
          if (bhpUnit != value)
          {
               bhpUnit = value;
               Changed("BhtUnit");
          }
      }
}

public ObservableCollection<StageModel> StageModelList
{
  ...............
    // BHP is bound to property of StageModel object & it binds perfectly well
}

public ObservableCollection<string> InjItems { get; set; }

I tried different ways to set the ItemSource of the combobox, but cannot set it. I have several other Combo's in same pattern and is coded in same style. Collection like InjItems is bound to more than 1 combo.

Why am not able to bind InjItems to the Itemsource of Combobox & set its selected to BhpUnit property of the model ? Can anyone please help me. I believe if 1 Combo works properly then my all Combo can be set accordingly and make workable.

Thanks

Thanks
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