There might be a leak in the pneumatic tubing to the actuator, actuator in the door might leak or the pump is bad.

Does your vacuum pump, below the rear seat, work normally, i.e. when locking/unlocking run for few seconds, not more?

Check the tubing first for leaks and operation first by removing every tube from the pump manifold and sucking/pressurizing them individually.

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S320L -97