Setting isExpanded on TreeView in WPF

I'm having a problem implementing a search functionality in my TreeView on a WPF Project.

I used this guide to create a TreeView with ViewModels. I have used the TextSeachDemo and edited the Controls in a way that they that they fit my application. (Added the right Classes, more layers etc.)

Everything works fine, I get a structure with correct children and parents and the search function also works, as it finds the correct entry.

Problem now is: When i try to set the "IsExpanded" Property from code nothing happens. Debubgging shows me that the PropertyChanged event in the RaiseProperty Changed Method is always null.

In the test Project provided by Josh Smith, everything seems to be working fine.  The only significant difference that i could make out is that he set the datacontext in code while i did in the XAML:

Code from Josh Smith:

 public TextSearchDemoControl()
    {
        InitializeComponent();

        // Get raw family tree data from a database.
        Person rootPerson = Database.GetFamilyTree();

        // Create UI-friendly wrappers around the
        // raw data objects (i.e. the view-model).
        _familyTree = new FamilyTreeViewModel(rootPerson);

        // Let the UI bind to the view-model.
        base.DataContext = _familyTree;
    }

From the constructor from the MainViewModel (The ViewModel that handles the entire window):

List<FactoryItem> rootItems = _machineService.GetFactoryItems();
  FactoryTree = new FactoryTreeViewModel(rootItems);

Where as FactoryTree is a public Observable Property:

        public FactoryTreeViewModel FactoryTree
        {
            get
            {
                return _factoryTree;
            }

            set
            {
                if (_factoryTree == value)
                {
                    return;
                }

                _factoryTree = value;
                RaisePropertyChanged("FactoryTree");
            }
        }

which i bind the DataContext of the TreeView too (instead of in code as above):

<TreeView DataContext="{Binding FactoryTree}" ItemsSource="{Binding FirstGeneration}">

The other way around by the way, when i open a item via the GUI, the breakpoint on my property does trigger and raise an event. So the connection should be there!

Any ideas? Even if its very very dirty workaround i'd be happy!

Julian Schneider