If I use
RevokePrivilege(me, "invul");
into an ai it npc remain invul

If I do in ai:
start_script("new_script", me)

and into "new_script.src":
RevokePrivilege(me, "invul");
npc become vulnerable.

Why?

why not adding SysLog() to your function call to help to better detect the problem?

Shinigami