I have the Spotify Desktop Client in version 0.9.17.1.g9b85d436 running on Linux Mint 64 Bit, and the spotify app on android in version 126.96.36.1998
When I add local mp3 files to my playlist in the desktop client, I can play them. When I open the playlist on my android app while the desktop app is running, and set the playlist as "available offline", the app makes all spotify songs available offline, but not my local files.
The device is shown in the desktop app under devices, and the playlist is also selected for syncing under "Devices / Nexus 4".
But on both the desktop, as well as on the smartphone, it says "sync pending".
This used to work well in the past. Any ideas?