Only two things I can think of. 1 there is a bad connection somewhere in there so you tapping is tripping the sensor into working. 2 sensor has its detection set distance set to zero...
Other than that you could try re-pairing the dodgy one without deleting it from the hub.. But if its 1 it won't make a difference. You might have just got a bad unit so I'd maybe think about returning it.
2 you can enable advanced options I think it is and check minimum detect distance on one of the working units and compare with the dud, they should be the same...