Have you changed countries? If yes then that's your answer. Not all items are available in all countries.
"Redownload your apps, music, movies, TV shows, and books on any devices that you might use to play or read them in the future. Some types of content might not be available in your new country or region. (Learn what types of content and services are available in your new country or region. - Availability of Apple Media Services - Apple Support )"
If you have not changed countries then it is possible the items were pulled from the store by the distributor. It is critical that you download everything and keep your own independent backups if you wish to guarantee you always have a copy. Re-downloading is simply a convenience but not something upon which you should rely.
Legal - Apple Media Services - Apple
- H. ”You can see Content types available for Redownload in your Home Country at Availability of Apple Media Services - Apple Support . Content may not be available for Redownload if that Content is no longer offered on our Services.
Content also may be removed from our Services at any time (for instance, because Apple loses its right from the Content provider to make it available), after which it cannot be downloaded, redownloaded, or otherwise accessed from Apple. We encourage you to back up your Content regularly."