Unable to Genrate an event for DropDown list in gridView

protected void GridViewDepartment_RowUpdating(object sender, GridViewUpdateEventArgs e)
{
getConnection = connection.SetConnection();
GridView GridViewDepartment = (GridView)sender;
GridViewDepartment.EditIndex = e.RowIndex;
GridViewDepartment.DataSource = departmentTable;
GridViewDepartment.DataBind();
int DepartmentID = (int)GridViewDepartment.DataKeys[e.RowIndex].Value;
DropDownList ddlManager = ((DropDownList)GridViewDepartment.Rows[e.RowIndex].FindControl("ddlDepartManager"));
string AssignNewManagr = ddlManager.SelectedItem.Text.ToString(); // this selecting only "select" 0th item only even on changing index
string AssignNewActive = ((DropDownList)GridViewDepartment.Rows[e.RowIndex].FindControl("DropDownListActive")).SelectedItem.ToString();
string SPUPDateDepartment = "SPModifyDepartment";
SqlCommand UPDateCommnad = new SqlCommand(SPUPDateDepartment, getConnection);
UPDateCommnad.CommandType = CommandType.StoredProcedure;
UPDateCommnad.Parameters.AddWithValue("@DepartmentId", DepartmentID);
UPDateCommnad.Parameters.AddWithValue("@DepartmentManager", AssignNewManagr);
UPDateCommnad.Parameters.AddWithValue("@Active", AssignNewActive);
UPDateCommnad.ExecuteNonQuery();
GridViewDepartment.EditIndex = -1;
GetDepartmentList();

}