Title "tooltip" not visible when hovering over images

I’m working on a website for my boss and have run into a problem.

He would like a brief description of a project to pop up when hovering over thumbnail images, instead of using the caption function. I have the title and alt text populated for each image, but the title “tooltip” doesn’t appear. (see this page as an example https://www.didierdesignstudio.com/national/ ) I understand that the alt attribute is not visible to browsers after researching the issue on the forums, but the title one should be.

I did find a plugin that worked, but it made navigation on mobile devices disappear, so I disabled it. The problem seems to be related to the theme, but I can’t find where. I set up a child theme for pilot fish to customize per his needs.

When I look in firebug, the alt text attribute is there, but not the title attribute (even though it is when I edit each image individually).

Any help would be greatly appreciated….