The unit and the Bluetooth device are too far apart.
Move the Bluetooth device closer to the unit.
There is a device (such as microwave oven and wireless LAN) that outputs signals in the 2.4 GHz frequency band nearby.
Move the unit and the Bluetooth device away from those devices. If your wireless router supports 5 GHz band, establish a new connection in 5 GHz band.
The Bluetooth device does not support A2DP.
Use a Bluetooth device that supports A2DP.
The connection information registered on the Bluetooth device is not working for some reason.
Delete the connection information on the Bluetooth device, and then establish a connection between the Bluetooth device and the unit again. For details, see the following: