My set up is this.  I have emitters creating particles.   I am trying to create a third emitter that gives off a burst of particles one time.  My emitter is set up to be a box.  At 2 seconds I have a key frame that has particles per second to be zero, next frame I make another key frame to emit 1000 particles set to emit once.  I get no particles.  If I set it to emit continuously it works fine.  There is something about the emit once setting that is not allowing any particles to be born.