Ford Focus 03 can unlock with central locking but will not lock sounds like it locks then un locks I have to hold key till door locks then lock passenger door the alarm then arms.Looked in Haynes thinking relay or master switch.?
I have seen similar issues when there is a break in the wiring loom so locking the drivers door doesn't cause the passenger door to lock, a common place to look is in the connection between the door (more likely the drivers side) and the bulkhead.