Unoconv is working well for us, however every once in a while a file will fail conversion.  The only way to get the submission to reattempt the conversion is to edit the submission, delete the problem file, reupload the file.

The Reset button in the Grading interface is nice for undoing unsaved changes, but a full reset would also be useful, that clears feedback, grades, comments, and deletes the existing pdf record and adhoc convert the file again.