poor volume control range

hi folks,
i am having an issue with my theremini.
even i calibrate 1ft range between low position and high position, volume don t change much and the suddenly, when my hand reach 5cm of the plate, bim the sound reduce on 2cm range from max to null
is it normal behaviour? how can i increase that?
thxx for your time

I am also facing the same here. Help is appreciated.

Thanks in advance.
Regards,
Shane