Tree view: just spins

When I use [wpdm_tree] or [wpdm_tree download_link=1] to create a tree view of all my docs, the folders with nothing in them open up just fine. But the ones that have content change to a “spinner” instead of the folder, and they spin but never display the contained documents.

I wonder if this is because my download files are PDFs instead of images? Is there a way to fix it?