Thumbnails don'r appear

What may be the reason for thumbnail images not to appear? -

Most of the items are PDFs, but this one is a JPG image -

My first guess is a problem with how Omeka and imagemagick are interacting on the server. First thing to check is to go to the General Settings on the admin side, and do the test at the bottom of the page to test the ImageMagick Directory path. If that doesn't work, that would be the quickest explanation.

If that's the case, your server admin or hosting company should be able to help you find the correct path. An alternative solution might be to change the configuration to use a different derivative images strategy. Depending on what's available on the server, a different strategy there might get better results.

If one of those approaches works for some new files, you can use the Derivative Images plugin to try regenerating the thumbnails.

Thank you! The ImageMagick Directory path was the problem in our case indeed.