When the letter was first sorted (either by Optical Character Recognition or, if that wouldn't work, by a postal worker) a series of machine-readable dots was printed onto the envelope, representing the postcode which the OCR device (or postal worker) thought was typed/written on the envelope. From then on it's those dots, rather than the typed or handwritten postcode, which the sorting machines read.
If you re-post the envelope the first sorting process sees those dots and assumes that the envelope has already been sorted on its current journey, so no new attempt is made to read the actual postcode on the envelope. You could re-post it a hundred times and exactly the same would happen, since nobody would ever read your message.
If you want to force the system to re-read the postcode, you need to cover up those dots (which can often be hard to see by eye).
Chris