epub 3 Ebook compile option not appearing for all files


I am in the midst of updating older book files, and am having some trouble with compile formats. For one of my files, I see “Project Formats” and “Scrivener Formats” under the “Formats” sidebar, and I am able to select “ePub 3 Ebook (.epub)” under the “Compile form” drop-down menu at the top when I try to compile. But when I go to compile other files, all I see under formats is “Default” “Ebook” “Ebook Screeenplay” and “Outline Document” under the “Formats” sidebar, and "ePub 3 Ebook (.epub) does not appeear under the “Compile for” drop down menu.

How can I make the epub 3 ebook option appear for all of my files?

Thanks in advance for any help you can give.

– Bliss B.