My iPhone 5 no longer shows up in the desktop sidebar under "Devices," so I can no longer sync my local files. I have Spotify Premium and have tried all the combinations of logging out/in, reinstalling, restarting, running as an admin, checking my firewall, etc. I'm on my university's network, so I have no control (to my knowledge) of how the internet works here.
There are probably restrictions in place which cause this. Unless you can persuade the network administrator to make the necessary changes, the solution would be to create an ad hoc wireless network on your PC and connect both the PC and your iphone to that. Synching should then be possible.