ComboBox ItemsSource binding not updated after value selected

I have a ComboBox in WPF binding its ItemsSource Property to a Property returning an IEnumerable of String. The binding is just one-way. The class that contains the data for the ComboBox implements INotifyPropertyChanged Interface and calls the OnPropertyChanged(..) as soon as the Property gets updated. When I leave the ComboBox untouched the changes are correctly propagated. But as soon as the ComboBox is expanded once or a value is selected the changes in the ItemsSource Collection are no longer updated. What may be the reason for this behaviour?

Heres the XAML

<ComboBox Name="cmbSourceNames" Grid.Row="0" SelectedItem="{Binding Path=CurrentSource, UpdateSourceTrigger=PropertyChanged}"
  ItemsSource="{Binding Path=SourceAddresses, NotifyOnSourceUpdated=True}"/>

The DataContext is set in the code behind:

this.cmbSourceNames.DataContext = this._dpa;

And this one is the Method that triggers the change of the Property. The Method for adding the Packet is delegated to the Current Dispatcher with BeginInvoke.

    private void DispatcherAddDataPacket(DataPacket dp)
    {
        ObservableCollection<DataPacket> dpList;
        this._dpkts.TryGetValue(dp.SourceAddress, out dpList);
        if (dpList == null)
        {
            dpList = new ObservableCollection<DataPacket>();
            dpList.Add(dp);
            this._dpkts.Add(dp.SourceAddress, dpList);
            OnPropertyChanged("SourceAddresses");
        }
        else
        {
            dpList.Add(dp);
        }
    }

The Property is giving back the Keys of the Dictionary as IEnumerable.