Paging doesn’t work?

Answered a question that’s virtually identical to this very recently on the WordPress.org forums.

Custom Post Type Archive Page – Pagination Isn’t Working

I’ve provided a work-around / fix in that thread that should also work for you to.

🙂

EDIT: You also need to add the paging parameter to your query..

Eg.

$args = array( 
    'post_type' => 'my_post_type', 
    'paged' => get_query_var('paged') ? get_query_var('paged') : 1,
    'posts_per_page' => 15, 
);
$loop = new WP_Query;
$loop->query( $args );

I reformatted the code to make it a little more readable, end result will be the same.