My armor and magic resist stop working

the first time this happen to me i was galio vs leblanc and she had zero magic pen but could nuke me i reported him for cheating witch i never do because the dmg was impossible. i will be playing a game and ppl will start ripping threw me as if there dmg is all true. thinking i might just b paranoid i asked a player what the base dmg on there spell was and it was even with the dmg i took even tho i had a full tank build trying to stop my champion from death. it is so bad i cant play ranked anymore because i might suddenly become useless.