Bad service

i went to kfc on the 28 may to buy 21 piece with my kids and i ask them with extra pack so that i can separate 5 piece because i don't stay with them the lady was a cashier that day said to me she can not help me because its not the first time she gave me the law of Kfc said they must not give me anymore .and i went and understand only to find out that when the kids got home it was 18 pieces not 21 and if the gave i guess I was gonna count it that time and decided not to go back because of the poor service and it was late and am not happy at all