Second picture of Thumbnail when cursing over is not always shown on the product page

Hi all. I have a problem with the thumbnails. When I am sliding over on the product page, he shows automatically a second thumbnail of the product. This is also the ideal situation, to show the second picture. But sometimes he is not doing it, randomly. Does anyone know how I can solve this problem?

Please have a look at our webshop yourself and see the difference at Aurum Ring 10mm and Aurum Ring 3mm when sliding over.

Aurum (ehinger-schwarz.de)