Cannot Set Random Order in query_posts

I’ve done this countless times in the past but can’t seem to get it to work on the site I’m building at the minute

<?php query_posts('post_type=slider&orderby=rand&posts_per_page=1'); ?>

It simply just loads the same post every time (there are 3 in total)

Can anyone see what I’m doing wrong? I do have Post Types Order plugin installed but I’ve disabled ‘auto sort’ and am still having the same problem