Three main factors can cause this and there are three possible countermeasures.
■ When the connection timing is the cause
If a mobile device that is out of charge is plugged into the DDJ-FLX4 for charging, it may not be able to connect. In this case, disconnect the USB cable from the DDJ-FLX4 and reconnect it.
■ When USB and Bluetooth connections are established at the same time
If your mobile device is connected via Bluetooth and your PC/Mac is connected to the unit using a USB cable at the same time, you may not be able to control the app correctly.
Quit all applications on the PC/Mac, disconnect the USB cable from the PC/Mac, and reconnect via Bluetooth again.
■ When the system environment is the cause
When using a Bluetooth connection, the following measures may work.
・If devices generating electromagnetic waves in the 2.4 GHz band (such as a microwave oven, wireless LAN or another Bluetooth device) are located nearby, move them away from the DDJ-FLX4 or stop using them (turn them off).
・ If your mobile device is far away, move it closer to the DDJ-FLX4.
*If powered speakers or headphones are connected via Bluetooth or have the surround feature, the sound will be delayed.