Been having this problem for several weeks now and its starting to do my head in. I think if it continues ill just end my premium account as there is no real point in keeping it.
So ive set up my music to download via wifi so i can play it offline, it all downloads to an SD card. This all works fine until spotify suddenly decides that this music is no longer downloaded (even though the files are still present on the SD card). If i start to resync it saves to my phone memory instead. The only way around this is to uninstall spotify, format the SD card, reinstall spotify and start the sync process again.
I've got the same problem on my Android-Smartphone. Seems like Spotify decides randomly a day per week "Oh, all music files are there. I don't like them, so I don't know them any longer.". Generally i like the app, but I'm not amused to download my whole offline music library (~ 18 GB) four times per month or even more often. Please fix this bug or make a workaround to make the Spotify App knowing it's files again. This is really going on my nerves.