Usually, this means you are incorrectly configured. Be sure to select only PCM 2.1 in the audio settings of your optical source. (TV/blue-ray) If it is set to Dolby Digital, it will not work well.
A sound bar is a 2.1 device, so if you feed it 5.1 signaling, it can act as the left/right speakers, not the center. In 5.1, the left/right speakers do not have voice.