Problems binding same datatype inf deep collection with HierarchicalDataTemplate - only displays first level

Hello together,

I am trying to bind my class HierarchicLabel to a treeview. The children are hosted in the Children property.

public class HierarchicLabel : INotifyPropertyChanged
{
// Constructors ...

    string _name;
    public string Name
    {
        get { return _name; }
        private set 
        { 
            _name = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Name"));
        }
    }

    List<HierarchicLabel> _children = null;

    internal List<HierarchicLabel> Children
    {
        get { return _children; }
        set
        {
            _children = value;
            if (PropertyChanged != null)
                PropertyChanged(this, new PropertyChangedEventArgs("Children"));
        }
    }

// .... more class stuff

My Binding to the treeview's ItemsSource is in code setting a List<HierarchicLabel> with the "top" instances of my class holding the rest of the items as children. Therefore, I used a HierarchicalDataTemplate:

<HierarchicalDataTemplate x:Key="HeadingTemplate" ItemsSource="{Binding Children}"><TextBlock Text="{Binding Name}"/></HierarchicalDataTemplate>

My TreeView looks like this:

<TreeView HorizontalContentAlignment="Stretch"
x:Name="references"
ScrollViewer.HorizontalScrollBarVisibility="Disabled"
ItemContainerStyle="{StaticResource TreeViewItemStyle1}"
Style="{DynamicResource TreeViewWord}"
ItemTemplate="{StaticResource HeadingTemplate}"
Background="x:Null}"
AllowDrop="True"
DragLeave="references_DragLeave"></TreeView>

When I bind my data, I expect an output e.g. like this:

1. Title 1

> 1.1. Subtitle 1

> 1.2. Subtitle 2

   > 1.2.1. SubSubTitle 1

2. Title 2

etc...

Here is my problem:

My TreeView only displays the first level (so like 1. Title 1, 2. Title 2). I cannot understand why. I read multiple examples binding data with a hierarchicaldatatemplate like this and it was said it worked fine, but it does not work for me. If this helps, I am using .NET 4.5. I also deleted all styles and templates from the treeview. So please can someone give me a hint, what I am doing wrong?

Thank you for your help.