Well I am now having this problem as well.

I have Spotify Premium v 0.9.1.53 desktop and 0.6.4.47 on my mobile. Using a Samsung Galaxy S3 running Android version 4.1.2.

Spotify found my phone with no problem until the last couple of days. Now it does not show it as a device when either when connected on the same wifi network nor even when connected via USB.

I am able to download playlists, however, I am NOT able to sync tracks from my own computer to my mobile.

I have tried rebooting both the phone and the computer and even uninstalling and re-downloading the app for mobile (but not yet for the PC). No luck.

Every thread I find comes back to "they need to be on the same network". They are.

Also, I have tried the preview link. That now only shows a work around for Linux.

Please help.