is the following line missing in your code?
while ( have_rows('your_repeater_row') ) : the_row();
so you may want to try like that to get the sub field:
<?php while ( have_rows('your_repeater_row') ) : the_row();
$attachment_id = get_sub_field('photo');
$custom_feature = wp_get_attachment_image_src( $attachment_id, "slide-thumb" );
?>
<img src="<?php echo $custom_feature[0]; ?>" />
<?php endwhile; ?>