Diagnosis
The green light means it has power and is paired, so the wireless link is good. The lack of bass usually means the soundbar isn't sending a low-frequency signal, which is often a settings issue.
How to Fix It
Grab your TV remote. Go into your TV's audio output settings and make sure it's set to "PCM" or "LPCM," not "Auto" or "Bitstream." The S400 can get confused by some surround sound formats and just stops sending bass to the sub. Also, press the 'Sound Mode' button on the soundbar remote until you see 'STANDARD' or 'MOVIE' on the display—avoid 'MUSIC' or 'CLEAR VOICE' which reduce bass.
If That Didn't Work
Perform a factory reset on the soundbar. Hold down the 'Volume -' and 'Input' buttons on the actual soundbar (not the remote) for about 5 seconds until the display flashes. This resets all audio processing.
Heads Up
Test with a movie explosion or a deep bass YouTube test video. Some TV news or podcasts have almost no low-end signal to send, so the sub will naturally be quiet.