caniknow

I took out a Klein NCVT3 tester out of packaging and was trying to test against electrical outlets, I get beep when getting the tester against a wall or a wooden table, but against the plastic casing that is by the side of switch i do not get a warning or a beep? Is there any calibration or something that needs to be done on this one? This is NCVT3 tester and receptacle tester combo that is available in home depot.