How to work with DropDown_SelectedIndexChanged Event for every time when the selected index is change

See more:

C#2.0

C#

ASP.NET

ASP.NET4

My Code:

SqlDBAccess db = new SqlDBAccess();
        protected void drpFaculty_SelectedIndexChanged(object sender, EventArgs e)
        {
            drpAdvisiorName.Items.Clear();
            drpAdvisiorName.Items.Add("<<Select Supervisor Name>>");
            try
            {
                db.Adapter("select TeacherName from tblTeacherAndDepartment where Department = '"+drpFaculty.SelectedItem.Text.ToString()+"'","teacherName");
                //lblerror.Text = drpFaculty.SelectedItem.Text.ToString();
                
                for (int i = 0; i < db.dataSet.Tables["teacherName"].Rows.Count; i++)
                {
                    //RadioButtonList1.Items.Add(db.dataSet.Tables["teacherName"].Rows[i][0].ToString());
                    drpAdvisiorName.Items.Add(db.dataSet.Tables["teacherName"].Rows[i][0].ToString());
                    //.Items.Add(db.dataSet.Tables["teacherName"].Rows[i][0].ToString());
                }

                db.dataSet.Tables["teacherName"].Clear();
            }
            catch (Exception ex)
            {
                lblerror.Text = ex.Message.ToString();
            }
        }

My ASP.NET Code:

<asp:DropDownList ID="drpFaculty" runat="server"

                   style="top: 83px; left: 240px; position: absolute; height: 26px; width: 473px"
                   AutoPostBack="True" onselectedindexchanged="drpFaculty_SelectedIndexChanged">
                   <asp:ListItem>Agricultural Engineering and Technology</asp:ListItem>
                   <asp:ListItem>Veterinary Science</asp:ListItem>
                   <asp:ListItem>Agriculture</asp:ListItem>
                   <asp:ListItem>Animal Husbandry</asp:ListItem>
                   <asp:ListItem>Agricultural Economics & Rural Sociology</asp:ListItem>
                   <asp:ListItem>Fisheries</asp:ListItem>
                   <asp:ListItem>Social Science</asp:ListItem>
                   <asp:ListItem>Nutrition and Food Science</asp:ListItem>
                   <asp:ListItem>BAM</asp:ListItem>
                   <asp:ListItem>ANSVM</asp:ListItem>
                   <asp:ListItem>Arts and Humanities</asp:ListItem>
                   <asp:ListItem>Biological Science</asp:ListItem>
                   <asp:ListItem>LAW</asp:ListItem>
                   <asp:ListItem>Electrical and Computer Engineering</asp:ListItem>
                   <asp:ListItem>Mechanical</asp:ListItem>
                   <asp:ListItem>Civil Engineering</asp:ListItem>
                   <asp:ListItem>Science</asp:ListItem>
               </asp:DropDownList>

My Problem:
When I am changing in the dropdownlist items than the value is adding in another dropdownlist only 1st-time. but, when I am changing second time the value of second dropdownlist is not changing. Why this is ? Is the event fired only 1st time? Or I am using ScriptPanale thus why the problem. Please check me out.
Thank you....