If you are using WP Engine and you want to load a random banner on every page refresh you may only get the same banner getting loaded all the time. This is caused because on WP Engine the database query ORDER BY RAND() has been disabled by default.
It’s easy to activate this function, just go to Admin Dashboard >> WP Engine >> General Settings Or directly enter this URL from your browser: http://yourwebsite.com/wp-admin/admin.php?page=wpengine-common