Without clues, it’s tough to guess what’s happening 
But I guess the native import/export plugin (this plugin is used to generated the preserved content) is failing and “swallowing” the errors (this has been improved in recent versions of OJS).
Was your installation updated from an OJS 2.x version? If yes, I’ve seen one common issue happening in the wild.
If this statement returns something, then you’re a victim:
SELECT * FROM submission_files WHERE file_stage IN (7, 8)
The fix would be to run (before running, better to know what you’re doing/fixing by reading this issue Fair Copy files not migrated when upgrading from OJS 2 to 3 · Issue #7470 · pkp/pkp-lib · GitHub):
UPDATE submission_files SET file_stage = 6 WHERE file_stage = 7;
UPDATE submission_files SET file_stage = 9 WHERE file_stage = 8;
Best,
Jonas