Local files show up as synced but are not available

[wolflambert]

I'm having an issue with Spotify Premium on a Nexus 5 with Android 5.0.1. I've synced quite a lot of local files and that usually goes fine. But two files suddenly aren't playable anymore ("Broken Jaw" and "Chin Music…"). They're greyed out and if I press to play them they're "not available". But this just happened when at school without an internet connection; first they were there, now they aren't. It's not like that with every file; "Ruby" is a local file, too, that one is perfectly fine and playable.

I thought, let's just resync them, but that doesn't work, because the tracks show up as already downloaded. New local files I add will sync, but not the unavailable ones. How can I resolve this? (The files are both linked to the catalogue; they exist on an edition of the album that's not available in my country.)