How can I run TV sound through my Samsung home cinema?

The surround sound works when playing DVD's but when it comes to watching TV through my Sky box I can't get it to work without unpluging and replugging the optical cable. Which takes numerous attempts.
Optical cable runs from sky box to system and HDMI cable runs from system to TV. I select the D'In source and the correct HDMI output from the TV but nothing!
Any help please?