wp_get_attachment_image_src image size

Hi

I can’t work out how to display the correct image size using wp_get_attachment_image_src. I’ve added the image size to functions.php and it is called ‘side’.

<?php $src = wp_get_attachment_image_src( get_post_thumbnail_id($item->ID), false, 'side' ); ?>

<img class="img-responsive" src="<?php echo $src[0]; ?>">

To me this looks like it should logically work, but I’m obviously missing something. It echos out the full size image.