The oxygen detectors sold by Analox for use with the LC panels use electrochemical sensors. These are really good at giving fast and accurate responses, however, they will also deplete over time. As they deplete their outputs relative to the oxygen detected will fall which means that the reading at the LC panel will fall too.
This is why Analox recommend periodic calibration adjustment of the oxygen sensors as detailed in the software manual. Wherever possible, it is advised that the sensors are first low (or zero) calibrated (we suggest a 0% O2 calibration gas such as 100% N2) before being high (or span) calibrated with as an O2 concentration as possible (we recommend 100% O2 but this can be lower dependent on your application).
If the sensor fails to calibrate, or continues to read low after carrying out the calibration adjustment, the likelihood is that the oxygen sensor needs to be replaced with new as it has depleted and is no longer functioning correctly.