it works for me :/
Could you get us a small capx ?
Edit : Never told us it was as a family ? You should check each of the creatures , try "For Each Creatures"
Edit 2 : O.k , found it out !
System | For each Creatures
System | Creature.HP < Creature.Prev_HP
-- Creature | Set Prev_HP to HP
-- Creature | Spawn Blood