WPF C# Treeview HierarchicalDataTemplate Basic implementation

I know many threads have been opened and I all read those to help better understand how to use the treeview and still I'm unable to render the treeview properly if not at all.

I only succeed to render one level of the tree and I have 3 level.

My question is : What is wrong with my code below ?

Level 1 class :

public class ProductModel
{
    public decimal? id { get; set; }
    public string product_pn { get; set; }
    public string product_desc { get; set; }
    public BoardTypesModel product_board_type { get; set; }
}

Level 2 class :

public class BoardTypesModel
{
    public decimal? id { get; set; }
    public string board_type { get; set; }
    public string product_family { get; set; }
    public float board_length_inches { get; set; }
    public List<PulseCurrentModel> lsPulseCurrents { get; set; }
}

Level 3 class :

public class PulseCurrentModel
{
    public decimal? id { get; set; }
    public float voltage_setpoint { get; set; }
    public float nominal_current { get; set; }
    public float current_tolerance { get; set; }
    public float nominal_power { get; set; }
    public float power_tolerance { get; set; }
    public string test_type { get; set; }
    public int order_priority { get; set; }
    public decimal? board_type_id { get; set; }
}

In my viewmodel, I have the following line of interest which is initialized properly on modelview load and it works fine :

    private ObservableCollection<Models.ProductModel> _ocProducts;
    public ObservableCollection<Models.ProductModel> ocProducts
    {
        get { return _ocProducts; }
        set
        {
            _ocProducts = value;
            RaisePropertyChangedEvent("ocProducts");
        }
    }

Finally, my xaml treeview code to render this observable collection :

<TreeView DockPanel.Dock="Top" ItemsSource="{Binding ocProducts}">
    <TreeView.ItemTemplate>
        <HierarchicalDataTemplate ItemsSource="{Binding product_board_type}">
            <StackPanel Orientation="Horizontal">
                <TextBlock Text="{Binding product_pn}" />
                <TextBlock Text=" - " Foreground="Blue" />
                <TextBlock Text="{Binding product_desc}" Foreground="Blue" />
            </StackPanel>
            <HierarchicalDataTemplate.ItemTemplate>
                <HierarchicalDataTemplate ItemsSource="{Binding lsPulseCurrents}">
                    <StackPanel Orientation="Horizontal">
                        <TextBlock Text="{Binding board_type}" />
                        <TextBlock Text=" (" Foreground="Green" />
                        <TextBlock Text="{Binding board_length_inches}" Foreground="Green" />
                        <TextBlock Text=" inches) " Foreground="Green" />
                        <TextBlock Text="{Binding product_family}" Foreground="Green" />
                    </StackPanel>
                    <!--<HierarchicalDataTemplate.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Orientation="Horizontal">
                                <TextBlock Text="Voltage : "></TextBlock>
                                <TextBlock Text="{Binding voltage_setpoint}" />
                            </StackPanel>
                        </DataTemplate>
                    </HierarchicalDataTemplate.ItemTemplate>-->
                </HierarchicalDataTemplate>
            </HierarchicalDataTemplate.ItemTemplate>
        </HierarchicalDataTemplate>
    </TreeView.ItemTemplate>
</TreeView>