Playlist artwork

Playlist artwork seems to be very inconsistent with local files for me. Sometimes it's there, sometimes it's not. For example, I have a 2 CD set that I ripped today.

I ripped the songs from the first disc, put them in a folder, added the folder to iTunes and added my own artwork in iTunes. The songs showed up in Spotify with artwork intact. When I added them to a playlist, the playlist too had artwork.

I did the exact same thing for the second CD, only, despite the artwork showing up for the songs on the second CD, the artwork would not display on the playlist. Why?