After dynamically adding TextBox and Label, then the DropDownList control's SelectedIndexChanged event is not working

protected void drpUniversity_SelectedIndexChanged(object sender, EventArgs e)
        {
            if (IsPostBack)
            {
                if (drpUniversity.SelectedItem.Value.ToString() == "Others".ToString())
                {
                    int rowcount = mytable.Rows.Count;
                    HtmlTableRow row = new HtmlTableRow();
                    row.ID = "tbl_row" + (rowcount + 1);
                    HtmlTableCell cell1 = new HtmlTableCell();
                    cell1.ID = "tbl_cell1" + (rowcount + 1);
                    HtmlTableCell cell2 = new HtmlTableCell();
                    cell2.ID = "tbl_cell2" + (rowcount + 1);
                    cell1.Attributes["class"] = "contact";
                    cell2.Attributes["class"] = "contact";
                    TextBox tb = new TextBox();
                    tb.ID = "tbQty" + (rowcount + 1);
                    tb.Width = 276;
                    Label lblotherUniver = new Label();
                    lblotherUniver.ID = "lbluniversity";
                    lblotherUniver.Text = "University Name";
                    cell2.Controls.Add(lblotherUniver);
                    cell1.Controls.Add(tb);
                    row.Cells.Add(cell2);
                    row.Cells.Add(cell1);
                    mytable.Rows.Insert(8, row);
                    mytable.DataBind();
                }
            }
        }