Can this be changed?

Is one being used for surround speakers? Line in cannot function in that situation. 

John B has a point.

The Line-In jacks include a switch to detect when a plug is inserted. Make sure that the plug is fully inserted. Also, it is possible that the RCA plug’s center pin is too short to activate the switch. Finally, make sure that you are using the correct input jacks. It’s easy to mix them up in the dark (been there, done that).

Hi John. I tried what you suggested and now working fine. Many thanks.