I had one problem with a prof. who agreed in the first place (months before the deadlines) to send out some letters on my behalf. He didn't respond to any of my reminders but managed to send at least one letter. When I kept asking right before the other programs deadlines, he responded with an angry mail. He said I shouldn't terrorize him with with my reminders and that he's busy. So I had to replace him last minute.
This particular prof. has still two days to send his last two letters. He made everything before christmas, but forgot two recommendations. Now, I keep reminding him daily as its already way behind the official deadline and the admissions officers told that the review starts this friday. No respond, no action. I guess those two apps are for the bin...