Thumbnails work on some pdf files but not others

Some of my pdf files generate thumbnails without an issue, and some don't. Here's a link to my site.

The PDF files were batch encoded by an outside contractor before I joined the library. The 2006 and later issues of the paper generate thumbnails just fine, and the 1999 and earlier issues generate thumbnails without a problem, but the issues from 2000 to 2005 all fail to generate any thumbnails.

Any idea on why I'm getting this failure?

Just upgrading Omeka may resolve your problem. Omeka 2.3.1 fixed some errors with making thumbnails that could arise particularly when working with PDFs.

So, I'd try upgrading Omeka to 2.3.1 or 2.4, and then making a test upload of one of the non-working PDFs. If that gets a correct thumbnail, you should be able to fix them all, either through reuploading or by using the Derivative Images plugin to regenerate the thumbnails.