How about a standardised preview for protected pdf files in lists.
Would it make easier to notice why a file has no preview.
Couple of tips:
 Check your Log file after import, as you may see information about files that couldn’t be correctly imported.
 After imports (especially useful for batches), select Tools > History. Create a new group anywhere you like in your database. Back to the History panel, select all the items within an appropriate date range, then use Control-click and choose Replicate To that target group. Now sort by file size. This provides quick information about empty documents.