DataBoundコレクションにアイテムを追加するときにビューが更新されないのはなぜですか？

私の簡略化されたコードは次のようになります。

モデル

class Playlist : INotifyPropertyChanged
{
    public event PropertyChangedEventHandler PropertyChanged;
    private void NotifyPropertyChanged(string PropertyName)
    {
        if (PropertyChanged != null)
            PropertyChanged(this, new PropertyChangedEventArgs(PropertyName));
    }

    private ObservableCollection<Track> _tracks;
    public ObservableCollection<Track> Tracks
    {
        get
        {
            return _tracks;
        }
        set
        {
            _tracks = value;
            NotifyPropertyChanged("Tracks");
        }
    }
}

ViewModel

class HubPageViewModel : INotifyPropertyChanged
{
    private Playlist _currentPlaylist;

    public event PropertyChangedEventHandler PropertyChanged;

    private void NotifyPropertyChanged(String info)
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(info));
        }
    }

    public Playlist CurrentPlaylist
    {
        get
        {
            return _currentPlaylist;
        }
        set
        {
            _currentPlaylist = value;
            NotifyPropertyChanged("CurrentPlaylist");
        }
    }

    public HubPageViewModel()
    {
        _currentPlaylist = new Playlist();
    }
}

見る

public sealed partial class HubPage : Page
{
    private HubPageViewModel _hubPageViewModel;

    public HubPageViewModel HubPageViewModel
    { 
        get
        {
            return _hubPageViewModel;
        }
    }

    public HubPage()
    {
        this.InitializeComponent();

        viewModel = new HubPageViewModel();
    }
}

XAML

<Page
x:Class="MyProject.HubPage"
DataContext="{Binding HubPageViewModel , RelativeSource={RelativeSource Self}}"
mc:Ignorable="d">

<snip>

    <HubSection x:Uid="HubSection5" Header="Now Playing"
                    DataContext="{Binding CurrentPlaylist}" HeaderTemplate="{ThemeResource HubSectionHeaderTemplate}">
            <DataTemplate>
                <ListView 
                    AutomationProperties.AutomationId="ItemListViewSection5"
                    AutomationProperties.Name="Items In Group"
                    SelectionMode="None"
                    IsItemClickEnabled="True"
                    ItemsSource="{Binding Tracks}"
                    ItemClick="ItemView_ItemClick"
                    ContinuumNavigationTransitionInfo.ExitElementContainer="True">
                    <ListView.ItemTemplate>
                        <DataTemplate>
                            <StackPanel Margin="0,0,0,27.5" Holding="StackPanel_Holding">
                                <TextBlock Text="{Binding Title}" Style="{ThemeResource ListViewItemTextBlockStyle}" />
                            </StackPanel>
                        </DataTemplate>
                    </ListView.ItemTemplate>
                </ListView>
            </DataTemplate>
        </HubSection>
 <snip>

なぜビューの更新は、ときに私は、追加または削除されませんTrack私にPlaylist？