We're running into this on certain assignments. In a given class one assignment is fine, PDFs show up, and another assignment in the same class PDFs are all blank for grading.
We're using the Google Drive renderer. It verifies fine (and is working since some assignments are just fine).
We're running moodle 3.3.1+ 20170817
Is there any place to find some error logging that isn't obvious? Debugging mode yields no additional info. It's like a silent fail.