Windows 7 - WMC7 Extender probs

Hi all,
Having issues extending wmc7. I'm running Windows 7 build 7022 with no probs, PC is connected to my network via powerline adapters as are other PC's. I have an xbox360 next to my htpc in the living room and wmc7 extends to this no problem (direct connection). I have another xbox360 in our bedroom also connected by powerline adapter, this box can connect to my network and to xbox live no problem but will not connect to my htpc downstairs either through the dashboard or Media Centre!!. I have all sharing options on, firewall off, and have entered the connection key several times but 7mc always fails at step 2 (searching for extender). Any help or ideas greatly appreciated.
thanks settingsun
P.S I'm just installing vista on another partition in order to test this but I think it will be fine.

Are the xboxes connected wired or wirelessly? Couldn't tell from your original message.

What happens if you unplug the first xbox and try again? Can you connect two seperate xboxes to the same PC/WMC? (not even sure if this is capable or not)