Mqtt ubidots receive datas

I’ve used ubidots.iot’s driver to communicate with mqtt. I’ve used publish function and it works. How can i receive values from ubidots?
I’ve used on_variable_update() but it doesn’t work.

Are you using the default example of ubidots software driver?
Could you tell me the result on the console when you change the variable value? if any.
Which of the supported boards are you working on?

I’m working on Esp32 DevKitC. I used the default example but I receive data only on the power on. I receive the last message sent when the esp32 was shut down. Yesterday it worked for me, today it exits MQTT CONNECTION ERROR

To better debug this.
Could you post the whole console output, from Power on to the error?