Was the sub-woofer included with the sound bar? I believe so but the Owner's Manual is very much mute regarding the sub-woofer.
Doublecheck the wiring connections per Page 13 of the Owner's Manual.
And, per Page 7, on the Soundbar Remote there are Volume + and - buttons for the speakers and Woofer Level + and - buttons for the Subwoofer' volume.
Take a look at Pages 21 and 22 regarding sound adjustment and troubleshooting. Take a look at the sound effect section and the note just below.
The goal is to eliminate some misconnection/wiring issue or sound configuration error.
Then the problem may be a defect in the sound bar, the sub-woofer, or the remote.