If your preview feature doesn’t work on your WordPress website for fields you created with the Advanced Custom Fields plugin (meaning you made changes to custom fields but after clicking preview you don’t see these changes) here is the solution.
Some hosts, such as WPEngine disable post revisions by default. The Advanced Custom Fields plugin uses this functionality for the preview of their fields. To resolve this open the
wp-config.php file on your server and replace:
define( 'WP_POST_REVISIONS', FALSE );
define( 'WP_POST_REVISIONS', 3 );
Now you should be able to preview your page and see changes made to your Advanced Custom Fields.