Messages droped without error message

I'm using this python module to send messages to a MQTT broker in Home Assistant. The code regarding the MQTT client is the following:
```
topic = '/homeassistant/zone_clusters'
client = mqtt_client.Client(client_id)
client.username_pw_set(username, password)
client.connect(broker, port)

time.sleep(row_time)
client.publish_qos = 2
for _, row in zone_clusters_df.iterrows():
json_row = row.to_json()
(rc, mid) = client.publish(topic=topic, payload=json_row, qos=2)
if rc != 0:
print(rc + ' - ' + mid)
client.loop()
time.sleep(row_time)
client.disconnect()
print('zone_clusters - done')`
```

But some of the messages are dropped without warning. I can see that by counting the number of messages received by listening to the MQTT topic (using node red).

Am I doing anything wrong in the code? If I'm using qos 2, shouldn't I receive at least a warning about a dropped message?
Thank you.