I can't find an exact answer to this question.
Playlists sync without problem, but I run into big problems if I update a file. As an example, lets say I rip a song from vinyl (I buy some Vinyl only releases) and I sync the mp3 to a playlist from my laptop to my iPhone (iOS 6). At this stage everything is fine.
But sometimes the volume of the MP3 is too low so I use Audacity to "amplify" the track. I save, overwriting the mp3 file and go back to Spotify on my laptop and mobile. I don't exxpect Spotify to notice the file update, but I remove the track from the playlist on my phone and on my laptop. I restart the mobile app and re-add the track but Spotify just uses the previous version of the track.
I've tried different orders of removing the track, restarting Spotify on my phone and on my laptop (Windows 7). I've even tried restarting my phone completely. Spotify just keeps holding onto the old version of the file.
What do I have to do to force it to update?