Header Highlight Content spacing

Hi, Referring to Theholyfish.com , when some posts are included in the header highlight content it creates gaps in the grid, such that some grid spots are empty while the posts are displaced onto lower rows. I've checked the size of the featured image for problematic posts and they appear correct. I read another forum post where font size was mentioned, but it doesn't look like the font is spilling over here. When I view the page in Wordpress 'customise appearance', the grid is displayed correctly. Thanks, Mat.