Down/stays are advanced obedience work and takes lots and lots of practice. You 'proof' the down/stays during training with low distractions....then medium...then high (mailman is high).
There is another element to this training. She somehow thinks it's her job to take care of the mailman. You teach her that it's not her job by getting between her and the mailman....showing her that you're handling the 'problem'. Make her back-up slightly...that tells you that she got the message. Eventually, of course, you'd like her to just go directly to her mat or rug and hold the down/stay when the mailman comes.
There is another element to this training. She somehow thinks it's her job to take care of the mailman. You teach her that it's not her job by getting between her and the mailman....showing her that you're handling the 'problem'. Make her back-up slightly...that tells you that she got the message. Eventually, of course, you'd like her to just go directly to her mat or rug and hold the down/stay when the mailman comes.