C#.net cookie經常性的丟失

這是我的源代碼，請教一下高手，為什麼經常會丟失這個cookie的值呢？

HttpCookie cookieUrl = new HttpCookie("User");
cookieUrl.HttpOnly = true;
cookieUrl.Expires = DateTime.Now.AddDays(1);
cookieUrl.Values["BackUrl"] = backUrl;
cookieUrl.Path = "/";
HttpContext.Current.Response.AppendCookie(cookieUrl);

這兩種方法都試了，都不能正常，開始正常，過幾分鐘就沒有了，有的時候用戶一登錄，馬上就提示未登錄，

HttpContext.Current.Response.Cookies["User"].HttpOnly = true ;
HttpContext.Current.Response.Cookies["User"].Expires = DateTime.Now.AddDays(1); //1天後過期
HttpContext.Current.Response.Cookies["User"].Values["BackUrl"] = backUrl;
HttpContext.Current.Response.Cookies["User"].Path = "/";