Centre speaker has no sound

For the past couple of years I used the pre-amp output for the centre and sent it back thru the TV. Now I have a new Plasma, I found it is now slightly out of synch so I bought a proper centre speaker and set it up, but I get no sound. I tested the speaker and it work fine, but I still get nothing when I connect it up to the centre speaker connections. I've adjusted the levels but still nothing.

Any idea what the problem is?

Shane