Particle spawn off screen

I ran into a weird problem with particles. I have an explosion that is triggered off screen. I'm using an isometric camera. The explosion has multiple emitters, one of them being a horizontal billboard (shockwave). The particles seem to spawn correctly except the shockwave, which spawn when i pan the camera so that the explosion is on screen. The shockwave should have died a long ago, but seems to spawn the effect even when the explosion has happened long ago. Other parts of the explosion seem to work correctly. Any advise?