Bose Lifestyle 12 - No centre sound in surround mode?

The system will kind of go into surround mode.
Back and front speakers work ok
Centre speaker has no output but if you listen up close, there is a slight hiss so there seems to be a connection but no audio, not even faint

The problem occurred after switching back from stereo to surround via the remote

I changed the batteries in the remote but it made no difference

Any ideas?

Thanks

Mike