TV won't work as external monitor with iMac core duo

Hi, I have one of the older white plastic imacs, a core 2 duo ca 2006 running OS 10.6.8, and I'm trying to hook it up to an HD TV using the VGA port. The cable's fine, and the TV works with my macbook pro as well as with my ipad, but when I plug the VGA with mini adapter into the back of the imac, the TV gets nothing. Any ideas on what to do about this?

Thanks!