Decorator Pattern - Access a wrapped ConcreteComponent Value

I currently working on an example using the Decorator Pattern. My current code looks like this:

abstract class LibraryItem

    {
        private int _numCopies;

        // Property

        public int NumCopies
        {
            get { return _numCopies; }
            set { _numCopies = value; }
        }

        public abstract void Display();
    }

    /// <summary>

    /// The 'ConcreteComponent' class

    /// </summary>

    class Book : LibraryItem

    {
        private string _author;
        private string _title;

        // Constructor

        public Book(string author, string title, int numCopies)
        {
            this._author = author;
            this._title = title;
            this.NumCopies = numCopies;
        }

        public override void Display()
        {
            Console.WriteLine("\nBook ------ ");
            Console.WriteLine(" Author: {0}", _author);
            Console.WriteLine(" Title: {0}", _title);
            Console.WriteLine(" # Copies: {0}", NumCopies);
        }
    }

    /// <summary>

    /// The 'ConcreteComponent' class

    /// </summary>

    class Video : LibraryItem

    {
        private string _director;
        private string _title;
        private int _playTime;

        // Constructor

        public Video(string director, string title,
          int numCopies, int playTime)
        {
            this._director = director;
            this._title = title;
            this.NumCopies = numCopies;
            this._playTime = playTime;
        }

        public override void Display()
        {
            Console.WriteLine("3");
            
            Console.WriteLine("\nVideo ----- ");
            Console.WriteLine(" Director: {0}", _director);
            Console.WriteLine(" Title: {0}", _title);
            Console.WriteLine(" # Copies: {0}", NumCopies);
            Console.WriteLine(" Playtime: {0}\n", _playTime);
        }
    }

    /// <summary>

    /// The 'Decorator' abstract class

    /// </summary>

    abstract class Decorator : LibraryItem

    {
        protected LibraryItem libraryItem;

        // Constructor

        public Decorator(LibraryItem libraryItem)
        {
            this.libraryItem = libraryItem;
        }

        public override void Display()
        {
            Console.WriteLine("2");
            libraryItem.Display();
        }
    }

    /// <summary>

    /// The 'ConcreteDecorator' class

    /// </summary>

    class Borrowable : Decorator

    {
        protected List<string> borrowers = new List<string>();

        // Constructor

        public Borrowable(LibraryItem libraryItem)
          : base(libraryItem)
        {
        }

        public void BorrowItem(string name)
        {
            borrowers.Add(name);
            libraryItem.NumCopies--;
        }

        public void ReturnItem(string name)
        {
            borrowers.Remove(name);
            libraryItem.NumCopies++;
        }

        public override void Display()
        {
            Console.WriteLine("1");
            // base.Display();
            this.libraryItem.Display();
            
            foreach (string borrower in borrowers)
            {
                Console.WriteLine(" borrower: " + borrower);
            }
        }
    }

Now I create a Borrowable-Decorated Video with the following code:

Video video = new Video("Spielberg", "Jaws", 23, 92);
Borrowable borrowvideo = new Borrowable(video);

What I want to do now, is to access the NumCopies - Property of the borrowvideo-object. The expected value is 23, as I specified that value in the video-constructor, but i get the value 0.

Can someone help me how to get the programm to return 23?

Thanks in advance!