Displacer level transmitter does not respond

respected sir,

calibration of oil/water interface displacer type transmitter by
weight and by using oil (0%) and water (100%) is perfectly ok .but when
kept on service, transmitter reading matching with respect to gauge
glass from 35 to 70% only, below 35% and above 70% transmitter not
responding.

replaced electronic head with torque tube, checked the displacer was
ok.

thanks & regards
krishna