Unable to save or update data in access database .. getting respose=0 after ExecuteNonQuery operation

private void btn_save_Click_1(object sender, EventArgs e)
       {
           string dt = Convert.ToDateTime(dateTimePicker1.Text.ToString()).ToShortDateString();
           int polfnum = 0;
           con.Close();
           try
           {
               string query = "SELECT COUNT(polf_number) FROM polf_details";
               con.Open();
               OleDbCommand cmd = new OleDbCommand(query, con);
               //OleDbDataReader oreader = cmd.ExecuteReader();

               //// Read the last value.
               //if (!oreader.Read())
               //{
               //       MessageBox.Show("Error reading from the table");
               //}
               //else
               //{
                     //polfnum = Convert.ToInt32(oreader.GetValue(1));
                     //polfnum = polfnum + 1;
                     // Display and Add it in the combobox for adding.
                     polfnum = (int)cmd.ExecuteScalar();
                     polfnum = polfnum - 1;
                     cb_polf_number.Items.Add(polfnum);
                     con.Close();
           //    }
           }
           catch (Exception ex)
           {
               MessageBox.Show("Error generating new number.\nError: " + ex.Message + "","ERROR");
               return;
           }
           string cust_name = cb_cust_name.Text.ToString();
           string circle = txt_circle.Text.ToString();
           int quantity = Convert.ToInt32(txt_qty.Text);
           string ponum = txt_po_num.Text.ToString();
           string artwrk = txt_artwork_name.Text.ToString();
           string moduletype = txt_module_type.Text.ToString();
           string product = txt_prod_name.Text.ToString();

           string SQLString = "INSERT INTO polf_details(polf_number, polf_date, customer_name, circle, quantity, po_number, artwork_no, module_type, product_type) VALUES('" + polfnum + "','" + dt + "','" + cust_name + "','" + circle + "', " + quantity + ",'" + ponum + "','" + artwrk + "','" + moduletype + "','" + product + "');";
           con.Open();
           //OleDbDataAdapter dAdapter = new OleDbDataAdapter(SQLString, connString);
           //create a command builder
           OleDbCommand cBuilder = new OleDbCommand(SQLString,con);

           int response = -1;
           try
           {
               response = cBuilder.ExecuteNonQuery();
           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.Message);
           }
           if (response >= 1) MessageBox.Show("Entry is added to database", "Successful", MessageBoxButtons.OK, MessageBoxIcon.Information);
           txt_circle.Clear();
           txt_qty.Clear();
           txt_po_num.Clear();
           txt_artwork_name.Clear();
           txt_module_type.Clear();
           txt_prod_name.Clear();
           cb_cust_name.ResetText();
           cb_polf_number.ResetText();
           datasaved = true;
           btn_new_customer.Enabled = true;
           btn_new_polf.Enabled = true;
           btn_save.Enabled = false;
           con.Close();
       }
   }

private void btn_update_Click(object sender, EventArgs e)
 {
     //sql query
     polf_details f1 = new polf_details();
     f1.con.Close();
     int polf_num=Convert.ToInt32( txt_polf_num_update.Text);
     string artwrk=txt_artwork_update.Text.ToString();
     string circle=txt_circle_update.Text.ToString();
     string module_type=txt_module_update.Text.ToString();
     string pono=txt_po_num_update.Text.ToString();
     string prod_type=txt_prod_name_update.Text.ToString();
     string cust_name=cb_customer_name_update.Text.ToString();
     int qty=Convert.ToInt32(txt_quantity_update.Text);
     string dt = Convert.ToDateTime(dateTimePicker_update.Text.ToString()).ToShortDateString();

     string SQLUpdateString = "UPDATE polf_details SET [polf_date] = @dt, [customer_name] = @cust_name, [circle]= @circle, [quantity]= @qty, [po_number]= @pono, [artwork_no]= @artwrk, [module_type]= @module_type, [product_type]=@prod_type WHERE [polf_number]= '"+ txt_po_num_update.Text+"'";

         OleDbCommand SQLCommand = new OleDbCommand();
         SQLCommand.CommandText = SQLUpdateString;
         SQLCommand.Parameters.AddWithValue("@dt", dt);
         SQLCommand.Parameters.AddWithValue("@cust_name", cust_name);
         SQLCommand.Parameters.AddWithValue("@circle", circle);
         SQLCommand.Parameters.AddWithValue("@quantity", qty);
         SQLCommand.Parameters.AddWithValue("@pono", pono);
         SQLCommand.Parameters.AddWithValue("@artwrk", artwrk);
         SQLCommand.Parameters.AddWithValue("@module_type", module_type);
         SQLCommand.Parameters.AddWithValue("@prod_type", prod_type);
         SQLCommand.Connection = f1.con;
         f1.con.Open();
         int response = SQLCommand.ExecuteNonQuery();
         if (response>=1) MessageBox.Show("Update successful!", "Message", MessageBoxButtons.OK, MessageBoxIcon.Information);
         Close();