This is my personal belive, I dont know how Decoys work.
After you drop a decoy he start to send messages to all missiles locked in your ship, the missiles AI gets confused and go after the decoy insted of your ship, but since the message is sent upon droping the decoy, new missiles fired at you dont get fooled since the AI of the new missiles is capable of ignoring the decoy message, however a new decoy will work on these missiles.
Well that is what I belive how they work...