dropdown list 3 ระดับ เลือกอำเภอแล้วแสดงตำบล ติดแค่ตำบลมันไม่ขึ้น

using System;
using System.Collections;
using System.Configuration;
using System.Data;
using System.Linq;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Xml.Linq;
using MySql.Data.MySqlClient;

namespace WebApp
{
    public partial class WebForm3 : System.Web.UI.Page
    {
        ClassConnectDB Connect = new ClassConnectDB();
        
        protected void Page_Load(object sender, EventArgs e)
        {

            // ดัก การ PostBack กลับ มาของ DropdownList1
            if (!Page.IsPostBack)
            {
                BindProvince();
                BindAmphur();
                BindDistrict();
            }
          
            
         }
     
        private void BindProvince()//bind   จังหวัดลง  student_province  dropdownlist
        {
            student_province.DataSource = getProvince();
            student_province.DataValueField = "Province_ID";
            student_province.DataTextField = "Province_Name";
            student_province.DataBind();
        }
        private void BindAmphur()//bind   อำเภอลง  student_amphur  dropdownlist
        {
            student_amphur.DataSource = getAmphur(Convert.ToInt32(student_province.SelectedItem.Value));
            student_amphur.DataValueField = "AMPHUR_ID";
            student_amphur.DataTextField = "AMPHUR_NAME";
            student_amphur.DataBind();

        }
        private void BindDistrict()//bind   ตำบลลง  student_district  dropdownlist
        {
            student_district.DataSource = getDistrict(Convert.ToInt32(student_amphur.SelectedItem.Value));
            student_district.DataValueField = "District_ID";
            student_district.DataTextField = "District_NAME";
            student_district.DataBind();

        }
        public DataSet getProvince()//ดึงข้อมูลจังหวัด
        {
            Connect.ConnectDB();
            string strSQL;
            strSQL= "select * from province";

          MySqlDataAdapter da = new MySqlDataAdapter(strSQL,Connect.objConn);
            DataSet ds = new DataSet();
            da.Fill(ds, "province");
            return ds;
        }
        public DataSet getAmphur(int provinceID) //ดึงข้อมูลอำเภอ
        {
            Connect.ConnectDB();
            string strSQL;
            strSQL = "select * from amphur where Province_ID=" + provinceID;
           
            MySqlDataAdapter da = new MySqlDataAdapter(strSQL,Connect.objConn);
            DataSet ds = new DataSet();
            da.Fill(ds, "amphur");
            return ds;
        }

        public DataSet getDistrict(int amphurID) //ดึงข้อมูลตำบล
        {
            Connect.ConnectDB();
            string strSQL;
            strSQL = "select * from district where AMPHUR_ID=" + amphurID;

            MySqlDataAdapter da = new MySqlDataAdapter(strSQL, Connect.objConn);
            DataSet ds = new DataSet();
            da.Fill(ds, "district");
            return ds;
        }

       
        //ตอนเลือกจังหวัด
        protected void student_province_SelectedIndexChanged(object sender, EventArgs e)
        {
            BindAmphur();//แสดงอำเภอ
           // BindDistrict();
        }
        //ตอนเลือกอำเภอ
        protected void student_amphur_SelectedIndexChanged(object sender, EventArgs e)
        {
            BindDistrict();//แสดงตำบล
        }
    }
}