iPod covers not showing in the main screen?

[ribbon]

I don't know why, but i've just realized that when I'm listening to music, if I press the button so "switch on" the screen while I'm listening to music, i see just the title of the song I'm listening to, but the cover does not appear. I have to unlock the phone to see the ipod app.

it does not happen with all the songs. it seems to be happening just with the new songs.

does anybody why can this be happening? this is my first time.

thanks in advance.