The OP is using powerline networking
http://computer.howstuffworks.com/power-network.htm
As I understand the power outlets in a room addition used to work - but not anymore.
I suggest to apply basic troubleshooting methodologies - it used to work, so what's changed? If nothing has changed then what is malfunctioning? Etc...
Try Googling powerline networking troubleshooting or similar and you'll find several items that may be of help.
I do computer networking for a living, specializing in corporate and carrier class wireless networks. I have never been a fan of powerline networking for the home...