silverlight treeview not loading subitems

I'm interested in finding out why this isn't working:

I have a treeview with some hierarchicaldatatemplates looking like this:

<UserControl.Resources>   
            <sdk:HierarchicalDataTemplate x:Key="nodeEntry">
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="{Binding Path=Title}" />
                </StackPanel>
            </sdk:HierarchicalDataTemplate>
            <sdk:HierarchicalDataTemplate x:Key="rootEntry"
ItemsSource="{Binding Path=Nodes}" ItemTemplate="{StaticResource nodeEntry}">
                <StackPanel Orientation="Horizontal">
                    <TextBlock Text="{Binding Path=Name}" />
                </StackPanel>
            </sdk:HierarchicalDataTemplate>
        </UserControl.Resources>

    <sdk:TreeView Height="250" HorizontalAlignment="Left"
    ItemTemplate="{StaticResource rootEntry}"
    ItemsSource="{Binding ElementName=subjectDomainDataSource, Path=Data}"
    Name="rootTreeView" VerticalAlignment="Top" Width="180"/>
The data is passed to the treeview from a domainservice using this method:

public IEnumerable<Subject> GetSubjectList(Guid userid)
    {
        DataLoadOptions loadopts = new DataLoadOptions();
        loadopts.LoadWith<Root>(s => s.Nodes);
        this.DataContext.LoadOptions = loadopts;
        return this.DataContext.Roots;
    }   
why then are only the root nodes shown in the treeview as if it only loaded a flat list, and not a hierarchy where the root Loads the NodesCollection?