...
“Dropped message: QoS 0 channel not writable” this error message is observed in the HiveMQ control center.
Answer
In general, this error happens due to clients not being able to read incoming messages fast enough. Please find further details on https://www.hivemq.com/docs/hivemq/4.9/control-center/analytic.html#channel-not-writable.
If This is not a broker error, as QoS 0 messages, according to the MQTT specification, have no delivery guarantee and can be dropped. The notification is displayed in the Control Center in order to notify about a possible issue with slow-consuming clients.
When your clients are consuming slowly then you can a possible solution is to use shared subscriptions to avoid this scenario or MQTT Addon topics for receiving dropped messages.