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Please read the support topic here for assistance setting up local file sync: Listen to local files
Make sure that you are on the same WiFi network and Windows & third party firewalls are configured to allow the Spotify connection through.
Another point to note is that sometimes if a track is stored locally & is also available to stream then when you sync to your device it will automatically use the Spotify streaming version but the data from your playlist. If for some reason the metadata such as track length differs this can cause conflict and break the track. In this instance you are best off removing the local file from your playlist on the desktop and adding the track from the Spotify server then re-sync. This should fix this issue.