R129 central locking

[dickyf]

i was hoping someone may be able to offer some assistance. the passenger door lock doesn't seem to be functioning correctly.
when inside with the doors locked i try to open the passenger door the auto feature doesn't work. If i open the drivers side first it releases the lock allowing the handle to be pulled and the door opened but it wont do it on the passenger side.
any ideas please. i can fix most things but cant work out what the problem is!

thanks in anticipation