that's weird... does it have a seperate preamp that has a vol control - I had one that used to "not work" then saw that it had its own control that I had to manipulate.
also, have you tried it without any of the other speakers on?
the way the human brain recognizes bass, it can't distinguish direction from it - so if you have the treble going on other speakers, regardless of where in the room your subwoofer is, it will sound like to you the deep sounds are coming from the smaller tweeter speakers.... weird brain trick.
this is why it doesn't matter where in the room you position the subwoofer - even under the couch.
if you turn off the other speakers, you can at least check that it is working.