Why does one person get shocked?
The shoes and socks can be leather and with sweat and conduct many uA of current. 


e.g. 230V/1M = 230 uA.  
dry socks and synthetic soles can be >>50M or 230V/50M=&lt;5uA  
If the insulation leakage in the machine is rated to 0.5mA then it's impedance is 230V/0.5mA = 460 kOhm 
so shoe interface impedance makes all the difference.


The machine has leakage current and 0.5mA (500 uA) can be felt but is generally considered safe and is the IEC/UL/CSA limit. It may be more and you ought to try measure it and fix it.