X1 box with optical cable loses sound

The arc output on my TV died, so I plugged an optical cable into the X1 box and connected it to my Polk soundbar that way. On half of the channels I'm getting Dolby and the sound is great, but on the other half the sound keeps going away. I would have to have the volume on max to hear it. Right now, for example, I have NBC on and can't hear a thing. Some of the commercials sound perfect and some have no sound, the news has no sound. I've tried both of the audio options for the Optical and nothing fixes this.