Question
“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 your clients are consuming slowly then you can use shared subscriptions to avoid this scenario or MQTT Addon topics for receiving dropped messages.