This is very strange. My Last payment is posted to the site but I haven't received it. Block explorer also shows "No such transaction" when I click on provided link in payment history. There seems to be a glitch in the system.
Have you read the help page that is shown after pressing the Instant Payment button ?
Or it was auto payment ? Don't worry, there are no glitches in payment system, it will reach you.
Hmm, I should invent a way to somehow show help page after autopayments too :)