Web Forms :: Fire Dropdownlist Selectindexchanged Event

I added dropdownlist & textbox control to gridviewrow dynamically on button click.I bind datatable to dropdownlist programatically & dropdownlist Datatextfield="product" & Datavaluefield="Amount". now my question is: when I select a product in dropdownlist,the corresponding product item "Amount" should display in the textbox. I write in my dropdown event like this:

protected void ddl1_SelectedIndexChanged(object sender, EventArgs e)
{
GridViewRow gridViewRow = (GridViewRow)(((Control)sender).NamingContainer);
DropDownList ddl1 = (DropDownList)gridViewRow.FindControl("ddl1");
TextBox itProductQuantity = (TextBox)gridViewRow.FindControl("itProductQuantity");
itProductQuantity.Text = ((DropDownList)sender).SelectedValue;
}

I call that textbox in Gridview row databound. why event is not firing?