Grey Boxes on Category display

Hello, I run a site that has hundreds of posts in different catigories. As such we have one page that loads with a dynamic serch depending on which place on the site you arrived at that one page. So for example if you clicked the link for easy activites, this page would only display easy activities.

For some of the catigories it displays them correctly in a nice picture grid.

Then for others each image has a large grey box below it and looks horrible. Not sure why this is doing it for some but not others as they are all functionally loading off the same page.

Could you please advise how to fix this?