I'm confused about why some pdf notes get rendered and some don't

My sincere apologies in advance if this has already been discussed or answered.

I am new to Joplin having fled from Evernote. I've exported/imported most of my Evernote notebooks into Joplin and have noticed something I don't understand: some of my imported pdf notes are previewed automatically in Joplin while others are not and instead just list the filename.

Clicking the filename opens the file externally without any problem, so the pdf isn't broken. And creating a new Joplin note from the same pdf (by drag/drop from the external file manager) creates a perfectly previewed note, again suggesting there's nothing wrong with the pdf. Finally, copying a previewable imported note from a different Joplin folder into this folder again leads to a perfectly previewed pdf, suggesting there's nothing wrong with the folder itself.

Any suggestions? Thanks in advance.

Never mind. This question has been superseded by much worse problems with syncing (see my other post to Support).