Can't sync local files to mobile (exceptional case)

[empoweredslave]

So I recently added local files of "A Blaze in the Northern Sky" album from Darkthrone to my computer, and I can perfectly listen to the songs on my PC. But when it comes to syncing it to mobile, I keep getting stuck at being able to listen to only one song out of 6. And that's because the only song that I can listen to in mobile is "In The Shadow Of The Horns", which is their only song from that album that is available on Spotify.

For solving the case, I tried creating a playlist that solely contains those 6 songs from that album, which works fine in computer, but again, in mobile, it shows that there are 6 songs in total in the playlist, but only one of them is shown, which is the aforementioned song. I also tried removing the song from the playlist and when I do that it shows 5 songs and none of them are available for listening. I'd also like to say that any other syncing attempt I've done in the past was succesful and this is the first one that's looking like a failure, for now.

Please help me with this problem tho, it's really f'in annoying, thanks!