WP_Query orderby rand isn't working?

Hello,

can anyone please advise why this is not working? It is just showing the same post each page refresh rather than a new random one from the list.

<?php $loop = new WP_Query( array(
'post_type' => 'testimonials',
'posts_per_page' => 1,
'orderby' => 'rand' ) );
?>

<?php while( $loop->have_posts() ) : $loop->the_post(); ?>

<div class="quote_bubble"><?php the_content(); ?></div>										

<div class="speech_bubble_credit"><?php the_field('testimonial_name'); ?><br />

<?php the_field('testimonial_job_company'); ?>, <?php the_field('testimonial_company'); ?></div>

<?php endwhile; wp_reset_query(); ?>

Any ideas anyone please?

Thanks!