Particular local song seems to be "downloaded" but isn't playing offline

[Aznaj]

Hi folks, 

got an annoying problem: I put some local songs in a playlist of mine, which are all playing perfectly in offline mode except for this ONE song, which isn't.

It's only playing in online mode with the desktop application running, else it won't.

The grey arrow of the whole playlist as well as of the song's green, so I would guess it's sort of "downloaded" but the song name itself is greyed out at the same time. I wouldn't make such a fuss about a single song but this is pretty much one I love, so, anybody got an idea how to fix this?   

Thanks in advance!