Problem locking my doors! Please help

Hi
i have a hyundai accent 99.I think I have central locking (im not sure what its called in english but its when u lock the all 4 doors at the same time). Anyway when I lock my car, only two doors get locked and I have to go to the other side of the car and lock the oher doors two but when I unlock it all the 4 doors get unlock. Why is it like this?
what to do?

thank u