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...
Ack. Sorry. Problem in my description. I mean if "ReservesNeedReturns" is turned "OFF" it will create problems, because $found will automatically be 'W' which will turn on $already_on_shelf, which means that _koha_notify_reserve is never triggered in Reserves.pm...
*** Bug 12010 has been marked as a duplicate of this bug. ***