INotifyPropertyChanged не обновляет UI (WPF)

Проблема в том, что привязка свойств ViewModel к свойствам элемента управления работает некорректно. Я проверил, что свойства и их значения меняются, но видимость элементов управления не меняется. Есть идеи, что с этим связано? Или я что-то упускаю?

ViewModel:

class MainViewModel
{
    public LoginViewModel LoginViewModel { get; set; }
    Notifier notifier = new Notifier();

    public MainViewModel()
    {
        LoginViewModel = new LoginViewModel();
    }
    private Visibility mdiPanelVisibility=Visibility.Visible;
    public Visibility MDIPanelVisibility
    {
        get
        {
            return mdiPanelVisibility;
        }
        set
        {
            mdiPanelVisibility = value;
            NotifyPropertyChanged("MDIPanelVisibility");
        }
    }

    private RelayCommand showMDIPanelCommand;
    public RelayCommand ShowMDIPanelCommand
    {
        get
        {
            return showMDIPanelCommand ??
                (showMDIPanelCommand = new RelayCommand(obj =>
                {
                    MDIPanelVisibility = Visibility.Visible;
                }));
        }
    }

    private RelayCommand hideMDIPanelCommand;
    public RelayCommand HideMDIPanelCommand
    {
        get
        {
            return hideMDIPanelCommand ??
                (hideMDIPanelCommand = new RelayCommand(obj =>
                {
                    MDIPanelVisibility = Visibility.Hidden;
                }));
        }
    }
    private event PropertyChangedEventHandler PropertyChanged;

    public void NotifyPropertyChanged([CallerMemberName] String propertyName = "")
    {
        if (PropertyChanged != null)
        {
            PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
        }
    }
}

и просмотр:

<Border Visibility = "{Binding MDIPanelVisibility}">
        <Border.InputBindings>
            <MouseBinding MouseAction = "LeftClick" Command = "{Binding HideMDIPanelCommand}"/>
        </Border.InputBindings>
    </Border>
    <ContentPresenter Width = "Auto" Grid.RowSpan = "2" Panel.ZIndex = "1" VerticalAlignment = "Center" Visibility = "{Binding MDIPanelVisibility}">
        <ContentPresenter.Content>
            <local:MDIView/>
        </ContentPresenter.Content>
    </ContentPresenter>
    <Button Content = "Личный кабинет" FontSize = "13" Command = "{Binding ShowMDIPanelCommand}">
        <Button.Style>
            <Style TargetType = "Button" BasedOn = "{StaticResource aLogButton}"/>
        </Button.Style>
    </Button>