I would love to help you out and I am sure others would love to, also. Though, can you please provide some more information on your setup such as Operating System, Which method you chose to connect it to your PC (audio connections), Your audio configuration in your Operating System, & etc?
Also, try to figure out when the sub is working. Such as only in video games, videos that are played within your web browser, playing mp3s, movie files.... and which media player do you use to play these mp3s/videos?