central locking problems with my 98 holden vectra?

i brought my car about 6 weeks ago and ever since ive had it ive had a few troubles with the central locking. on the key there is a button but some times the button works but sometimes it doesnt when it does it only unlocks the front 2 doors and the boot. the left side back door i had troubles with for about 3 weeks. the door unlocks fine when u pull the lock up but doesnt open, its like it isnt relasing it. the right back door sometimes thr lock will be stuck down and i cant pull it up to unlock it. i took my car back to the car yard and they pulled the door off and put a new lock thing on it but its stil playing up every now and then. does anyone know what i can do to fix it and also what is causing thuis problem. thanks