XF 2.2 lightbox behavior not consistent

re: photos attached to threads and mirrored in the media gallery:

New Threads widget is used to populate the home page, where the images appear.

sometimes clicking on the image on the home page opens up the lightbox. other times, the image has no link, so clicking on it does nothing; in which case, opening the thread reveals an image link; clicking on the image then shows the lightbox.

any ideas why this isn't consistent?