I've just bought Air Sync but DoubleTwist on my Windows7 PC can't see my Galaxy Nexus. I've already tried all of the steps in this guide: http://www.doubletwist.com/help/quest... .. but no luck.
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Another test that you can run is to install a FTP server on your Android device such as this one:
After installation and launch, it will ask you to setup a name/password combo (make sure "Accept connections from wifi" is enabled). Hit "save" and then "start". The Wifi URL will be displayed.
Enter this URL into either a FTP-aware browser or FTP client on your computer and see if you can login to your phone. If you can not then something is actively blocking communication between your phone and your computer. The first suspect would be the wireless router. We would suggest logging into the router's admin interface to see if certain firewall/filter settings such as as "AP isolation" has been enabled.