Yeah, it sounds to me like it's a firewall issue. Here at work, for instance, even though I have full network privileges, having the phone on the same WiFi as my computer doesn't allow me to use SideSync (something to do with the router's config, and I'm not the admin, so I couldn't tell exactly what). Pushbullet works becuase it's not really using a 'local' network. It's sending the info through Internet packets, which is why you don't even need to be in the same network as your phone to do this. SideSync, on the other hand, creates a local, private connection to your computer, which is why you have to be in the same network for this to work.