New Ipod Nano and cover flow: where are my covers?

[kebosma]

I just bought a new Ipod Nano, 8GB with cover flow.

When I enter the cover flow menu, most of my covers are not shown.

For numurous albums the cover suddenly pops up when I really start playing a song from that album, next to the song itself.

But for most of my covers, it will not be shown at all...

When my Ipod is in the main menu, it shows a slideshow of album covers next to the main menu, and there the covers are all shown, also the album covers which are not shown in cover flow... So the pics are there, why doesn't they appear in cover flow?

First I thought it had something to do with my own made covers, but I already saw that some covers I made are shown perfectly...

Anyone?