Button not showing up

I installed the plugin today. In my loop-single-post.php I have added this line:

<?php the_flattr_permalink() ?>

In the webpage, I see nothing. In the sources I get:

<a href="http://omegataupodcast.net/2013/09/130-violin-physics/" title=" ">

At the bottom of the page I get the JavaScript code that loads the API code
from http://api.flattr.com/js/0.6/load.js?mode=auto’

Any tipps on what could be wrong?

Thx,
Markus

http://wordpress.org/plugins/flattr/