It seems Curse is a little buggy. Some monster don't take damage when using attacks while cursed.
List of know effected monsters:
*LUMBERS never take damage from being cursed.
*RETRODES do not take damage from curse when using their melee attack. They DO take curse damage when using their laser attack, however.
If you know any other monsters that aren't properly effected by Curse, please post them here.
I've noticed this too but I assumed it was because curse may work on the monsters like it does the players. When a player gets cursed, not every attack and vial they have gets the curse status put on it. So if the weapon you're using is cursed, you simply change to one that isn't and continue attacking. I assumed that some enemies weren't ALWAYS taking damage everytime they attacked because maybe only one attack of theirs is cursed, because it does seem like enemies take damage when they do one attack but not another.