Works in Debug mode but not in release mode

This is a simple program I wrote to teach myself some C#. It is a lotto number picker that creates a game object which is an array of 6 numbers then, on pressing the 'Create' button, prints it out in a text box. The program works fine in debug mode but only displays the first set on numbers in release mode. Thanks for any help given.

public int NoOfGames { get; set; }
List<Game> myList = new List<Game>();

private void btnCreate_Click(object sender, EventArgs e)
{
    NoOfGames = (int)udNumber.Value;

    myList.Clear();
    txtList.Text = "";
    String vl = "";

    for (int i = 0; i < NoOfGames; i++)
    {
        Game game = new Game();
        game.ClearGame();
        game.FillGameArray();
        myList.Add(game);

        for (int j = 0; j < 6; j++)
            vl += String.Format("{0}, ", myList[i].gameArray[j]);

        vl += "\r\n";
    }
    txtList.Text = vl;
}