03 Kia sorento power door lock control will unlock doors, but not lock doors.

Just purchased a used 03 Kia Sorento. The powewr door lock button on the drivers side (the only one in the vehicle) will unlock all the doors, but will not lock any of the doors. When the switch is activated to lock the doors, nothing happens and a clicking noise (much like a relay) is heard underneath the drivers lower dash (just above the fuse box).