Alternative image hover is inconsistent

Struggling to find consistency in terms of hovering over items for an alternative image

I have created a number of product pages, all with only one image
Top Left image has been hovered over and shows a broken image link.  Others work as they should (like the sunflower), and don't try to show a second image.  

I can't see any differences in how these products/page/styles have been set up

Can anyone help, please?

Hi,

Can you share link to your site? We can take a look