Artist and album show as 'Unknown' - calling iTunes experts

Hi I have posted before on NAS / iTiunes and artwork issues. Basically I have given up trying to get artwork with from the WD mybook live via n-stream, and will eventually get a mac mini.

In the meantime my iTunes library on my NAS is no longer showing the name of either the artist or album name on my Qute display - all I seen for both is 'unknown' (the name of the track does show). However for some albums it is fine and everything shows correctly.

I had assumed this was a metadata or some folder issue at the iTunes end - but when I drag the album with artist and track details missing onto a flash drive it all does show up correctly. And when I compare albums that show correct data to those that don't I can't see any difference.

So I am a bit confused. I assume there are some folders 'pointing' incorrectly and it's not obvious. I run mavericks BTW.

Tips and thoughts welcome.

regards

M