Background Image not fixed on mobile devices

Hello, I have a Problem with the Background Setting of the Catch Responsive Pro Theme: What I did, was choosing a Background Image an picking the Setting "Screen filling". On my PC everything looks great and the Background Image ist "fixed" as i wanted it to be! Now I checked everything on my mobile device (IPhone 4s) and noticed that the Background Image isn't fixed there. The same problem on IPad and other Smartphones. Any advices please?