回发不会显示更改，直到有额外的回发

我有一个工作的登录表单。我只是想做一个简单的测试，看看它是否有效，它似乎只工作时张贴回第二次。换句话说，

我输入我的用户名/密码并按下登录按钮，它返回但没有显示我登录了。

再次点击按钮，重新加载等，它将再次发送所需的结果。

// from my pageload
//System.Threading.Thread.Sleep(new TimeSpan(0, 0, 0, 10)); 
//didn't work
if (PageBase.Account.AuthenticatedUser != null) //if user is logged in...
    formLogin.Attributes.Add("Style", "background-color:Green");
else
    formLogin.Attributes.Add("Style", "background-color:Red");
// just changed the background color. 2nd postback it turns green.

谢谢你的帮助和输入。

编辑:需要更多代码

public class AccountManager {
    public Website.User AuthenticatedUser {
        get {
            int id = Convert.ToInt32(( (object)HttpContext.Current.Session["user_id"] ?? 
                                       (object)(HttpContext.Current.Request.Cookies["USER_ID"] ??
                                                new HttpCookie("bugfixcookie") { Value = "0"}).Value));
            var user = (from u in new MyWebsiteEntities().Users where u.ID == id select u).FirstOrDefault();
            return user;
        }
    }
    // create user, 
    public bool Login(string username, string password, bool remember) {
        var result = (from u in new MyWebsiteEntities().Users
        where username == u.Username && password == u.Password select u).FirstOrDefault();
        if (result != null) {
            if (remember) HttpContext.Current.Response.Cookies.Add(new HttpCookie("USER_ID", result.ID.ToString()));
            HttpContext.Current.Session["user_id"] = result.ID.ToString();
            return true;
        } else return false;
    }
    public void Logout() {
        HttpContext.Current.Response.Cookies.Remove("USER_ID");
        HttpContext.Current.Session.Remove("user_id");
    }
}

和

public class PageBase : System.Web.UI.Page
{
    public static AccountManager Account { get { return new AccountManager(); } }
}

前端

<form runat="server" id="formLogin">
    <asp:TextBox runat="server" ID="textKey" style="display:none" />
    <asp:CheckBox runat="server" ID="checkboxRemember" />
    <div> 
        <span id="un">Username</span><br />
        <asp:TextBox runat="server" ID="textUsername" />
    </div>
    <div> 
        <span id="pw">Password</span><br />
        <asp:TextBox runat="server" ID="textPassword" TextMode="Password" />
    </div>
    <asp:Button runat="server" ID="buttonLogin" OnClick="buttonLogin_click" />
    <asp:Button runat="server" ID="buttonRegister" OnClick="buttonRegister_click" />
</form>

再次感谢!