Revisons visable but not clickable

I need to revert back to an older page. Revisions have been selected within the screen options and are visable as a list on the edit page. The problem is that the list items are grey and not clickable. I’ve looked at wp-config.php hoping to find define(‘WP_POST_REVISIONS’, false ); which I would change to “true” but this wasn’t present. I’ve read through documentation which suggests by default this is ‘true’ anyway.

I’m a little confused. Could a plugin cause a problem?

Any help would be greatly received.

Tayler