iTunes 10.7.0.21 no longer 'gets album artwork'

It used to be the case until very recently, that you could always get the album artwork for tracks either imported from CD, or bought from other stores by choosing 'Get album artwork' from the right click menu, and sometimes you had to modify the name of the artist or album in order for it to match the iTunes one, but it doesn't work at all anymore. It always comes back with 'Artwork for xxxx could not be found'.

Has anyone else come accross this, and might know why, and how to fix it?

Many thanks in advance.