Put the ignition on second switch so you have dash lights,put the car in reverse and rub your finger across the sensors gently and you should be able to feel them pulsing if one isnt then there is your problem,if none are then it could have a system that if one doesnt then they all dont work which could mean wiring or the main parking sensor unit,
If your getting a contstant noise like it is picking up an object again this could be because one isnt fitting flush and is picking up park of its own surround,we had a car with front sensors doing this ages ago which had been to the dealers and alsorts before i fiqured out the front number plate tape was lifting and the inner sensor was picking this up with it been stuck proud of the grille by a couple of mm