Does Home Sharing only work over wi-fi?

This may seem like an odd question, but I think it may be the root of my problem. I have a network that uses three Devolo mains network extenders in various rooms (the house has very thick walls). All three Devolo mains plugs connect (via the ring main) to the router. An iMac, laptops, an Apple TV, iPhones, etc each connect wirelessly to these very happily.

However, no two computers (say an MBA and an iMac) in the same room, or different rooms, can 'see' the other in the normal way - i.e. by having the other device appear in the left hand pane of the finder window. They can, however, if I enter the IP address of the other machine manually. So this is irritating but not insurmountable.

What does appear insurountable though is that, in a similar way, iTunes cannot 'see' any shared libraries across the network. Because of the issue described above, I had assumed that both problems were probably being caused by some sort of issue with the way the Devolo boxes are configured. But then I realised that, even though each device is connecting wirelessly to the Devolo boxes, I don't actually have a wi-fi network in the same sense that I would if I had an Airport-based system...and if Home Sharing only works over wi-fi, this might explain why it doesn't work for me.

Apologies for the rambling question, but given the increasing popularity of these mains network-extender boxes, I feel I can't be alone in experiencing these issues. Can anyone shed any light?