NPCs have to be set specifically to be able to be raised/resurrected. This is best set in the creatures OnSpawn script. The command you need is:
void SetIsDestroyable(int bDestroyable, int bRaiseable = TRUE, int bSelectableWhenDead = FALSE);
Parameters:
bDestroyable
If this is FALSE, the caller does not fade out on death, but sticks around as a corpse.
bRaiseable
If this is TRUE, the caller can be raised via resurrection. (Default: TRUE)
bSelectableWhenDead
If this is TRUE, the caller is selectable after death. (Default: FALSE)