Hmm ok, let’s debug.
In your wp-show-posts.php file, find this line: https://github.com/tomusborne/wp-show-posts/blob/1.1.3/wp-show-posts.php#L383
Right after it, add this:
Now on your site you’ll see all of the args printed. Do you see your custom query in there? (you may need to ctrl + f).