Is your WordPress theme not saving revisions for custom post type? The issue might be not declaring support for revisions in custom post type. Whenever you create a custom post type and you wish to have revisions, you need to manually declare it.
Find the code where custom post type is declared. It is usually located in the theme’s function file. And modify the code something like this –
‘supports’ => array( ‘title’, ‘editor’, ‘revisions‘ )
You are done.
Also, for testing purpose make some changes so revision meta box will appear.