[SwissXC90]

Has it rained recently? Mine stop working sometimes after rain, where the rear of the car has become very wet.

Try to extract any moisture out from the tiny gap between sensor and bumper. User tissue paper to draw out the moisture.

If that resolves it, water wicking in the gap is the problem, as this causes the ultrasonic pulses to be reflected back by the bumper itself.
I saw a service bulletin about this once... the permanent solution is some form of sealant or gel to stop water getting in the gap. I've never tried it.

Otherwise, a single sensor may be faulty. VIDA will tell you which one.
People with sensitive fingers say they can feel or hear the sensor pulsing, but I've never had luck with that. I must be too tough.