When running (nVision) Report Books, job runs to Success, but not all Reports generated?

We encounter an issue periodically, particularly during very busy reporting periods, when users will run Report Books containing multiple reports, the job runs to Success, but not all the reports are generated.  In this case, the users have access to the reports and can rerun the Report Book at a later time and the report generates without issue (sometimes with or without other reports within the Report Book).  The job log will throw an Excel interface exception for a very generic exception and there's no indication that the report fails, but it does not generate.    Has anyone encountered this issue?