iPod won't show album artwork on lock screen?

For some reason, on songs that I recently synced to my iPod, it won't show my album artwork on the lock screen. I have a 16gb 4th gen iPod touch. When im playing the song while it's unlocked, it shows the artwork. It also shows it in cover flow. But when i lock it, then press the home button, it shows my wallpaper instead of the album artwork. Why is this only happening to recently synced songs and not my synced songs from a while back?