C# - Dropdownlist Selectedindexchanged Event Doesnt Change Its State?

In the following code everytime its taking only one item from dropdownlist. When I select any other item from dropdownlist its same as first item.

protected void DropDownList1_SelectedIndexChanged(object sender, EventArgs e)
{
DataSet _subcat = new DataSet();
_subcat = serviceus.Get_SERVICEUS_SUB_CATEGORYLIST(DropDownList1.SelectedValue.ToString());
lbsubcategory.DataSource = _subcat.Tables[0].DefaultView;
lbsubcategory.DataTextField = Convert.ToString(_subcat.Tables[0].Columns["CATEGORY_SUB1_NAME"].ColumnName);
lbsubcategory.DataBind();
Label5.Visible = true;
}