iPod Nano 7th Album Artwork not shown

Hello.

Recently my iPod Nano 7th gen has stopped showing the artwork for new albums. It still showed the artworks for previously loaded albums, but in no way it was showing the new ones, even though they were fine on iTunes. (Note that I load music manually from my hard drive.)

I even reset the device to default settings twice, but that didn't work out; actually, now I've also lost the artwork for the albums that were shown before.

How can I solve this issue? Thanks in advance.