Album work from iPhone 4 won't show

So a couple days ago, I put some of my music onto iTunes and then put it on my iPhone. When I went to my Music, all the album art work won't show. Only the songs I had just downloaded and one song that was a single. Any way to put the art work back onto the music?

Try restoring it

Go to iTunes, right click the song, and click "get album artwork."
This usually happens to me when I download CD's onto it.