single pic shortcode don't display in category, how to fix it?

i have custom a category with content which load from a post id

$post_id = 926;
$post_object = get_post( $post_id );
echo $post_object->post_content;

but it don’t work in my site
http://hoangdatsg.com/category/thong-tin-san-pham/nguyen-vat-lieu-va-may/phu-kien-kim-khi/

but in content post id 926 is ok.

how to fix it?
tks for advance!

https://wordpress.org/plugins/nextgen-gallery/