Hello hippophile1 -
I'm sorry to hear that this difficulty has persisted for you.
Please be aware that if you revise your description on eBay, it is possible to damage the gallery code, and, if you use prior listings to create new listings using either the Saved Listing and then saving as New or using Create similar, the data from the initial listing is included when you begin working on the new listing. If there is an issue with the HTML in a listing you are using to make a new listing that issue is carried across to the new listing.
Please also note that if you have only one active item at any time, the gallery will not display on your listing as it would only be able to show the listing that it is on, but anytime you have more than one listing available the gallery should show with your items as defined in your gallery preferences.
Given your description here, it is unclear what may be occurring for you - however, if the issues persists, particularly in listings that have not been revised on eBay, we encourage you to continue with your support case with specific examples so we can look further into the HTML on the listing.