Thanks for the reply.
Please try testing for a plugin conflict. You can do this by deactivating all third party plugins, and seeing if the problem remains.
Before I try this, I think I may have determined the issue. If I load the page in-question and open up Chrome Dev Tools' console, there are seven errors. Four of them are of this form:
Access to Font at 'http://www.tidessamui.com/wp-content/plugins/media-grid/css/lcweb-media/fonts/lcweb-media.ttf' from origin 'http://www.deckchairstoreroom.com' has been blocked by CORS policy: No 'Access-Control-Allow-Origin' header is present on the requested resource. Origin 'http://www.deckchairstoreroom.com' is therefore not allowed access.
The other three are of the form:
Failed to load resource: the server responded with a status of 400 (Bad Request)
Do you think this is worth following up as the potential source of error? I should tell you that the site with the error is the private testing and development site that we use for our actual live site. The live site is www-dot-tidessamui-dotcom, and the dev site is www-dotdeckchairstoreroom-dot-com-slash-tidessamui. I tried to insert actual URLs there, but the editor gave me an error, saying new users could insert only two links per message.
I have created an admin user for you to use on our dev site. But before I share that with you in this post, I need to know if it will be private or not. How can I share sensitive information with you in this forum and ensure it is available to only you (and other devs)?
You can login at www-dot-deckchairstoreroom-dot-com-slash-tidessamui. You can view the gallery in-question by navigating to www-dot-deckchairstoreroom-dot-com-slash-tidessamui-slash-gallery and opening any of the galleries on the page.
If you want me to continue to troubleshoot plugin-by-plugin, I can--just let me know.
edit: it seems as if the console errors are related to the "Media Grid" plugin. Would you agree? Is that an X-Theme Extension or bundled plugin?