Why isn't my iPhone music showing it's album covers?

So I have purchased all of my music from my iPhone, but I decided to plug it into my computer yesterday so I could add an AC/DC CD that I bought. When the album was put onto my phone, only the AC/DC album covers showed up! Is there a solution?

1.) Open iTunes
2.) Click the "Settings" button at the top left corner
3.) Hover your clicker over the Library option
4.) Select "Get Album Artwork"
5.) Sync iPhone

This usually works for me!