Fullscreen not really fullscreen

The plug-in used to work perfectly for me. Until I had to back up my wordpress site and migrate to a new host. Nothing else has changed, and everything else on the site seems to be working fine. However when I click on the thumbnails, the translucent-black layer only covers about half the height of browser window, with the image fitting inside it.

See here: https://revantasarabhai.com/lucifer/

Is there some setting I need to change to make it cover the full size of the browser window?

Thanks.