Armor penetration

I was playing Custom game and i saw that armor penetration not working at all. I test it on monsters, lane minions and champions. When attack Monsters with armor, it dont reduce your dmg, that means if u attack monster with or without armor dmg will be the same. When u attack champion his armor reduce your dmg. Any way armorpenetration from runes and items not working in custom game. I didnt test it on ranked. I hope u all understund.