Missing songs in iCloud

I bought an album containing 20 songs back in 2012. The receipt in iTunes confirms 20 songs. However, when I look under "purchased" in iTunes, I only see 14 songs. Also, I can only download 14 songs on new devices. I clearly remember being able to play all 20 songs for a while. So either my "purchased" list in the cloud got corrupted at some point or Apple no longer has the missing songs (in which case I am afraid there is not much I can do). The album is no longer in the iTunes store.

Fortunately, I still have copies and backups of the original 20-song album, but I still would like to fix this.

Thanks in advance.