Problem with playing my Ipod through my stereo?

I have a Yamaha receiver R-S700 and a Yamaha dock YDS-12, both purchased in 2010. I had an Ipod nano from 2007 that fit into the dock and played through my stereo. Then the Ipod died. I had another nano from 2012 which also fit directly into the dock but would not play through the stereo. It plays in my car via the usb port and plays with an earpiece. So I bought a brand new Ipod nano, which has a "lightening" connection. Apple gave me an adapter so it would fit into my dock which it did. However, same problem as the 2012 nano, it would not play through the stereo but again plays in my car and with the ear piece.

I called Apple who had nothing to offer but the usual "it should work." I called Yamaha who said the same thing but finally offered that it must be the dock that went bad. (But if the dock went bad, why was it able to play my 2007 nano?) So I'm buying a new dock and will see what happems. In the interim, does anybody have any ideas?

Thank you for your help with this very frustrating problem.