Transactions do not work

hello everyone, someone will know me to say what am I wrong ...? transactions do not work or maybe something else ...
I expect that if there is an error in the insert the transaction does not even update ... instead update is always performed ...
what do you recommend?
I did a lot of tests trying also with manual transaction .. but nothing
thank you so much
Giuseppe
and sorry for my english !!

I attach the relevant code

PHP Code:

 $this->db->trans_start();

            foreach ($libriInVisione as $value) {

                $value = explode("|", $value);
                $idMagazzino = $value[0];
                $quantita = $value[1];

                $this->db->set('quantita', 'quantita - ' . $quantita, false);
                $this->db->set('data_modifica_riga', 'NOW()', FALSE);
                $this->db->where('id', $idMagazzino);
                $this->db->update('magazzino');


                $this->db->set('id_cliente', $idCliente);
                $this->db->set('id_rappresentante', $idRappresentante);
                $this->db->set('id_magazzino', $idMagazzino);
                $this->db->set('bolla_visione', $bollaVisione);
                $this->db->set('quantita', $quantita);
                $this->db->set('quantita_visione', $quantita);

                //$dataVisione = '23-5-2016 23:15:23';
                $dataVisione_formattata = date("Y-m-d", strtotime($dataVisione));

                $this->db->set('data_visione', $dataVisione_formattata);
                $this->db->set('data_inserimento_riga', 'NOW()', FALSE);
                $this->db->set('data_modifica_riga', 'NOW()', FALSE);
                $this->db->set('is_active', 1);
                $this->db->set('operatore', $this->session->userdata['name']);

                $this->db->insert('in_visione');
            }

        $this->db->trans_complete();

        if ($this->db->trans_status() === FALSE) {
            $this->db->trans_rollback();
        } else {
            $this->db->trans_commit();
        }