Are the Wireless ID Switches on the soundbar and the sub set to the same value (pages 4 and 5 of the owner's manual (PDF))?
Also, from the troubleshooting section on page 10:
No Sound from the Subwoofer.
• Check the points in the “Soundbar does not power on,” section above.
• Turn up the volume control.
• Check that the wireless ID setting is the same on the soundbar.
• Move the subwoofer closer to the soundbar.