通过MVVM绑定的observatecollection<T>不会更新视图

我有以下简化的视图模型

public class UserViewModel : IUserViewModel
{
    public DelegateCommand<object> NewUser { get; private set; }

    public ObservableCollection<UserInfo> UserList { get; set; }

    public UserViewModel( ) {
        NewUser = new DelegateCommand<object>( OnNewUser );

        this.UserList = new ObservableCollection<UserInfo>( );
        UserList.Add( new UserInfo( "Peter" );
        UserList.Add( new UserInfo( "Paul" );
        UserList.Add( new UserInfo( "Mary" );
    }

    private void OnNewUser( object parameter ) {
        UserInfo user = new UserInfo( "User"+DateTime.Now.ToLongTimeString(), 0, true, false, false );
        UserList.Add( user );
    }
}

此视图模型是(简化)视图的数据上下文:

public partial class UserView : ViewBase
{
    public UserView( UserViewModel viewModel ) {
    InitializeComponent( );

    this.DataContext = viewModel;
    }
}

视图本身包含一个绑定到用户视图模型的用户列表的列表框:

<ListBox ItemsSource="{Binding UserList}" DisplayMemberPath="UserID" />

viewModel.UserList.CollectionChanged += new System.Collections.Specialized.NotifyCollectionChangedEventHandler( UserList_CollectionChanged );

[编辑]

public class UserButtonBarViewModel : IUserButtonBarViewModel
{

    public UserButtonBarViewModel( IUserButtonBarView view, IUserViewModel mainModel ) {
        this.View = view;
        NewUser = mainModel.NewUser;
        this.View.Model = this;
    }

    public ICommand NewUser { get; private set; }

    public IUserButtonBarView View {get; private set; }
}

<UserControl ...>
    <Button Command="{Binding NewUser}" Content="New user"/>
</UserControl>

以及视图背后的代码:

public partial class UserButtonBarView : IUserButtonBarView
{

    public UserButtonBarView( ) {
        InitializeComponent( );
    }

    public IUserButtonBarViewModel Model {
        get {
            return DataContext as IUserButtonBarViewModel;
        }
        set {
            DataContext = value;
        }
    }
}

为什么不调用视图的处理程序?为什么列表框在视图模型中的集合更改时不更新自身?