Spinner.gif Not Showing While Waiting On Process


Right at the beginning of my project I cloned the responsive theme to my own theme. I don't know if I had it before that, but ever since I am working on this project the spinner.gif is not showing, just a semi-transparent black box in the center of my screen. All the processing, like add to cart, is working just fine.

The image itself is available on the remote server. So why is it not showing correctly? And how do I fix it, in which (core-)file?