There are so many possible reasons that I hate to hazard a guess. Instead, let’s turn on auditing and find out if the reports are being created, if they are being created – where, and if they’re being deleted, printed, moved, etc by whom. This link should help you get started with auditing on the system. The two audit values I believe you will be interested in are *SPLFDTA and *PRTDTA (though it never hurts to scan the others in case you could use them for another purpose some day 🙂 ).
I hope this helps,
I have no disagreement with auditing, but I might have gone with Job Accounting first. The print/spool entries in the system job accounting journal seem a little friendlier for this kind of tracking, but that’s purely opinion.