Since you only have one spawner and one timer, with multiple instances of trolls they are restarting the timer all the time.
You should probably move the Timer behavior to Troll object. When health<=0, instead of destroying this troll, hide it from the screen and start a timer on it for 2 seconds. On timer event, destroy this troll and spawn a new one. Or you can simply move it to spawner position and reset its health (make him alive again).