David, Gawain: IIRC, I've had Infomaker reports fail to email even though SSRS was working fine due to the Windows Print Spooler service having quit! Infomaker reports don't have native PDF export, so they use Ghostscript which relies on a virtual printer. (Fun Fact: the next version of PowerBuilder will have PDF generation built in! No idea if Tessitura is planning to update, though.)
As for random IM reports failing to email, I would really expect to see something in the logs; either the logfiles or the email log tables. But one thing you might want to check is that your Report Server concurrency is set to 1 (this is configured in the report server itself, or its ini file). Any other value makes it untrustworthy IMO -- I've had simultaneous schedules get their report attachments switched. That might be another thing to check -- are the reports that are failing to email scheduled at the same time as other schedules, or are they all spread out?