It might be in the scripting attached to the trolls. Are the trolls:
A) taking no damage at all?
taking damage but never becoming wounded?
C) taking damage but never getting below "near death"?
A might mean the script is setting the trolls to "plot", making them invulnerable, or they have serious damage resistance/immunity.
B might mean that the trolls are being hurt, but are just regenerating incredibly fast.
C might mean the trolls are set to "immortal", so they take damage but are never killed (unless hit with enough acid/fire damage for a certain script to fire off and remove the immortality).