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Drag all the songs from the Local Files or Songs into your Playlist, it will show you a small popup how many songs you currently have.
Are you trying to download on WiFi or mobile network? Are these local files you are trying to sync or are they SPotify stored tracks?
Does your device have more than 10% free space on the storage medium?
The toggle is swtiched on already. The downloading songs bar appears and disappear itself without downloading a single song which are left. NEED HELP!
I checked some possible solutions in the thread below.
And some users said that it got fixed when they uninstalled and reinstalled the app and then re-synced.
Also are you sure you don't have other devices with songs saved Offline? You might hit the 3.333 per device limit.
There you go, you reached the limit of 9.999 songs. In the meantime you can check out these Idea threads below and give some Kudos and leave a comment.