It looks like from your homepage you’re using Gridalicious from Catch Themes. If so, they provide great support on their site catchthemes.com and may have a better answer than me since they wrote it ??
That said, I’ve worked with their themes before and most of the time they set attachments to use the sidebar of the page/post they are attached to, so I’d make sure the attachment is attached to a post/page that has the sidebar you want. It appears that https://eliotterholt.com/photography/dsc03508-v3-3/#main is an attachment attached to a page and https://eliotterholt.com/shop/photographs/attachment/39/#main is a post so I’d check to see if your posts and pages are using the same sidebars or if there is a theme option to do that.
Otherwise try to recreate https://eliotterholt.com/photography/dsc03508-v3-3/#main as a post and see if that generates the sidebar you want.