The nice thing about powerlines is that you can quickly test what works and doesn't work, then swap to another wall-socket if those magical 3 lights don't come on. Quick, easy. Unfortunately, that "work or doesn't work" is also their dictum - when they don't, there's no setting to trick them into it!
Don't forget Bill's standard comment either: "directly into the wall socket, no power strips or extension cords."
Sometimes, I've re-located the Host unit away from the Computer Room nearer to the Target Receiver Room. Instead of running a longer cable in Mum's room, therefore, maybe I'd run a longer cable to some other room for that Host Unit's connection.