To solve the issue, try to turn off the printer and back on again. If the issue continues, check the following.
 

Is the media installed in the printer？

(For RJ-2035B/2055WB/3035B/3055WB only)
Is the printer offline?

The printer automatically turns off if it has not received any print jobs for 30 minutes. Turn the printer power ON or change the sleep mode time. For details, see SET STANDBYTIME or SET SLEEPTIME command in the FBPL Command Reference Manual on the [Manuals] section.

Is the printer paired with your computer?

From the computer's Settings menu, select Bluetooth setting.

Confirm that your computer pairs with the printer correctly.

(For RJ-2035B/3035B only)
Was the printer connected to your iOS/iPadOS device before?

• Turn off the Bluetooth setting on the device which is connected to the printer.
• Tap "Remove Device" of the printer you want to disconnect to from the list of Bluetooth device, and then disconnect it.

Has the printer's Bluetooth MAC Address changed from the default settings?

Confirm the BT MAC Address setting in the BPM (Brother Printer Management Tool).

For details, see the Bluetooth^® Connection (using the BPM) section of the FAQ: ”How to connect the printer to your computer (for Windows)”