Thank you very much for your quick reply

I am using Beez3 on a Joomla 3.9.28 site but I've just figured out that it was some customization code I wrote in a file I didn't check (while trying to see if that was the problem before posting), that was interfering with the additional images display.
By correcting it, it works perfectly now. I'm sorry for this mistake on my part.
I just have a last question if you don't mind. When I click on an additional image on the bottom of the main one on my site, it displays a fancy box to show it but on the VM demo's website, when I click on one additional image, it is displayed, not in a fancy box but take the place of the main image. I don't know if you understand what I mean but this is what happens on this page for example,
https://demo.virtuemart.net/default-product-detailHow can I move from this fancybox to the additional image going into the main image?
Thanks and have a nice day,
ml12