asp.net相关，三级联动，第三级菜单无显示，帮我看看是不是少了什么？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using LoginAndRegist.DAL;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace LoginAndRegist
{
public partial class index : System.Web.UI.Page
{
protected void Page_Load(object sender, EventArgs e)
{
if (Session["HasLogin"] == null || Convert.ToBoolean(Session["HasLogin"]) == false)
{
Response.Redirect("Login.aspx");
}
else if (Convert.ToBoolean(Session["HasLogin"]) == true)
{
if (!IsPostBack)
{
DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='1'"));

            ddlProvince.DataSource = dt;
            ddlProvince.DataTextField = "AreaName";
            ddlProvince.DataValueField = "AreaID";
            ddlProvince.DataBind();

            ListItem li = new ListItem("未选择", "0");
            ddlProvince.Items.Insert(0, li);
            ddlCity.Items.Insert(0, li);
            ddlCounty.Items.Insert(0, li);
        }
    }

}

protected void ddlProvince_SelectedIndexChanged(object sender, EventArgs e)
{
    DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='{0}'",ddlProvince.SelectedValue.ToString()));

    ddlCity.DataSource = dt;
    ddlCity.DataTextField = "AreaName";
    ddlCity.DataValueField = "AreaID";
    ddlCity.DataBind();

    ListItem li = new ListItem("未选择", "0");
    ddlCity.Items.Insert(0, li);

    ddlCounty.Items.Clear();
    ddlCounty.Items.Insert(0, li);
}

protected void ddlCity_SelectedIndexChanged(object sender, EventArgs e)
{
    DataTable dt = DBHelper.GetDataSet(string.Format("select * from AreaInfo where ParentID='{0}'",ddlCity.SelectedValue.ToString()));

    ddlCounty.DataSource = dt;
    ddlCounty.DataTextField = "AreaName";
    ddlCounty.DataValueField = "AreaID";
    ddlCounty.DataBind();

    ListItem li = new ListItem("未选择", "0");
    ddlCounty.Items.Insert(0, li);
}

}
}

根据你提供的代码，三级联动的实现似乎没有问题。可能是数据库中某些数据存在问题或缺失导致第三级菜单没有显示。建议检查数据库中相应数据是否存在，并且确保数据的格式与代码中的查询语句匹配。同时，你可以在代码中加入一些调试代码，以便更好地定位问题所在，比如在获取数据的地方加入一些输出代码，以确认是否成功获取数据。