You have to "pick" the instance you want to act on or else all instances will be picked.
In short you have to figure out a way to single out the instance you want to fire separate from the other instances.
In my prototype space shooter in my sig I used the technique mentioned by currypuff. My basic enemy ship each shoot at a random time.. it works for the most part. Some of my enemies, however, are picked by distance from player and others using a collision with an invisible sprite barrier... you have to figure out what works for your specific need.