I'm not sure why this happens. Have you tried uploading the gallery picture again in a couple of listings to see if that helps?
This isn't really related to your question but you should consider using ebay's gallery to host all of your pictures rather than putting them in the listing description. Often buyers do not scroll down to check the description area and if they do, they may not scroll past all of the pictures to see your item description.
Plus,some mobile users do not see that section at all unless they specifically click on the section to open it. More and more people are using mobiles to make online purchases so it is a good idea to make your listings more mobile friendly. There are other ways to do that but I'll just stick with mentioning the gallery pics for now. 🙂
To use that option on Auctiva - just below the image selection section, there is another section called Optional Ebay gallery images...click on the box under that to enable that option.