This issue occurs because the imported images don’t have two meta fields required for HivePress to detect them as a part of the image gallery:
hp_parent_model meta value set to listing
hp_parent_field meta value set to images
Please try contacting the import plugin support and ask them how to add meta to the imported attachments, this should resolve the issue. If there’s no such option (based on the screenshots above, they have options to set meta for the imported post/listing, but not the attachments), then the only way would be using another import plugin (you can try using our HivePress Import extension), or another possible option is creating a custom code snippet that sets the required meta, but this requires code tweaks.
Hope this helps