It is either, Bluetooth module does not fit in the connector correctly (old Bluetooth module)
or pairing info saving need to be cleared (new Bluetooth module)

To distinguish old/new Bluetooth module, you need to see firmware version.
Firmware version is on the top of self test.
So it is necessary to get self test working.

The way to perform the self test is as follows.
●Keep the LF switch pressed down.
●Press the ON/OFF switch down.
●Release the ON/OFF switch.
●Release the LF switch.

Firmware for old Bluetooth module : Up to 2.07
Firmware for new Bluetooth module: After 2.12

If you cannot get self test or your Bluetooth is old one,
I think you need to contact your dealer for the service.

If you see firmware version after 2.20, you can try followings.

Turn the printer off
Press ON/OFF button and keep it until LED becomes red Press ON/OFF once
Press LF twice
Press ON/OFF 5 times
Press LF once
(Printer turns off by itself)