Good morning Peter, if you have followed the instructions and set up the sensitivity levels correctly, then the detector should work satisfactorily. I have been able to detect timber studs to a couple of mm and cables going to a socket or light switch ok. If metal partition studs have been used this does cause a problem because the detector cannot isolate cables located closely to the metalwork. Try on a plasterboard wall both stud and dot’n’dab to detect a stud and then retry on your plaster wall. If this doesn’t work, consider returning the item to the seller as you cannot rule out that it could be faulty.