When you register your CPT, you need to manually add support for revisions. Only title and editor are set as defaults. Check out the [codex][1]. So if you need to enable revisions, you need to add it like this

`'supports' => array( 'title', 'editor', 'revisions' ),`

 in your `$args = array()` when registering your CPT.

Just for info, here are the other arguments you can pass to 'supports'

    'title'
    'editor' (content)
    'author'
    'thumbnail' (featured image, current theme must also support post-thumbnails)
    'excerpt'
    'trackbacks'
    'custom-fields'
    'comments' (also will see comment count balloon on edit screen)
    'revisions' (will store revisions)
    'page-attributes' (menu order, hierarchical must be true to show Parent option)
    'post-formats' add post formats, see Post Formats

**EDIT**

Thanks to @jammypeach, it is important to know, the revision metabox will not show up until there are actually revisions for the post. If there are no revisions, the revision meta box will not show

  [1]: http://codex.wordpress.org/Function_Reference/register_post_type