Media library not displaying thumbnails

I look after a site (https://www.farmandplantsales.co.uk)that invites users to add listings to sell tractors and other equipment. They can add images to these listings if they wish. Now that people have added these images, they are showing up in the media library but without a thumbnail attached (so I cannot work out which media item corresponds with which listing.

When in the list mode, the thumbnail is blank. In grid mode I get an image that looks like a page with the top right corner folded over.

When you click on edit on a particular item, the image is there.

Other images that I have uploaded (banner etc) are appearing correctly with a relevant thumbnail.