Issue with conductivity measurements

Hello,
I’ve been using a ProfiLux Plus II eX for several months, but I'm experiencing an issue with conductivity measurements. The readings are consistently too low.

I’m using an original GHL Conductance Platinum electrode. During calibration in a 50 mS solution, I obtain values of 19 for ADC1 and 382 for ADC2. However, after calibrating and leaving the electrode in the 50 mS solution, the device only displays 48.2 mS after a waiting period.

Initially, I thought the issue might resolve itself over time, but unfortunately, I've faced this problem since the very first day, and there has been no improvement.

What could be causing this?