Note: This is very Amazon, very MTurk and very specific to people who have joined Amazon Web Services as MTurk Requesters.
Apparently, requesters can only contact workers with whom they’ve already worked, i.e. approved or rejected past work. The API does not return errors when you attempt to notify workers without this relationship, but will not send the message.
I’m in contact with Amazon about the problem.
Original Post: here.
About Mechanical Turk: here.