The connection procedure may be the cause.

You can't establish Bluetooth connection with your mobile device from the setting window of the smartphone when you use to connect Bluetooth headphones, etc.

In this case, a message such as "Pairing Unsuccessful" or "Couldn't pair with DDJ-FLX4" is displayed.

Be sure to start the application before Bluetooth connection.

*You can't connect to a mobile device via Bluetooth automatically.