Can't add artwork to iTunes...

I have been adding art work gradually to my iTunes for the songs I ripped from my CDs. I was doing just fine by highlighting the entire album, right clicking, hitting "Get Info", and just adding the art work. But just recently it has decided that enough is enough, and will not add any more. In the art work box where a plus used to show up when you drag a picture over, now a circle with a slash through it appears. And I have tried this with all sorts of bands so I know it has nothing to do with a certain one. Plus the fact that I don't have any .WAV files on iTunes, so I have no idea what it could be.