probably the motor itself is broken or sticking. try lightly tapping the motor that actuates the locking machanism with the panel off, and pressing central locking button at same time. but probably you will keep having same problem you probably will need to replace little motor that actuates locking mechanism.
the problem with all electric systems is when they use sh1t motors which obviously break you are left with no manual over ride.
Liam
September 2009