If you have the system preference "ReservesNeedReturns" turned on, notices for "Holds Filled" won't be generated because the logic thinks that it has already sent one. As far as I can tell, "notificationdate" doesn't even appear to be used. That seems like it would be an excellent place to record the date that a notice was sent out...
*** This bug has been marked as a duplicate of bug 12011 ***