The issue is that you are running a custom query ($query_productos), but your next_posts_link is using information from the main query ($wp_query) to determine whether or not to show a link to the next page.
See the example on next_posts_link for use with WP_Query, by passing max_num_pages from your custom query –
<?php next_posts_link( 'Older posts', $query_productos->max_num_pages ); ?>