house Humidity too low - winter time

Problem: Humidity level inside is about 30% measured electronic and analog. Normal 'winter' level is 35-38%.

Setup: Aprilaire 700 and Trane basement unit (Nat Gas). Modern large house with a second parallel setup on second floor. Finished basement. There was a hose leak at the top of the humidifier pad that was repaired several years ago. Water valve to humidifier is shut off in the summer months. new pads installed at beginning of every season.

All components seem to work fine. A700 is set near high (humidity level), pad is wet, excess drains away thru tube. all confirmed visually. There is calcium build up in certain areas that are cleaned during summer time.

In January, (outside temps 0-32F) both thermostats are 'adjusted' to permit the basement unit to run longer (fewer start cycles), heat basement, heat rises to first floor and small amount of bonus heat to second floor. Second floor runs , but less frequently in this setup. Recent near zero temps for a week plus, resulted in basement heater running continuously for 20 out of 24 hours a day (not measured, but observed). low speed, 'low' heat most of the time. second floor thermostat would be adjusted down during the day since less activity there and this unit would run infrequently. During long heating runs by the basement unit, I would become aware that humidifier is engaging occasionally, but not as frequent as I would expect. I do not expect humidifier to run constantly. But, low measured humidity and less-frequent running makes me wonder what kind of sensor (electronic or physical) may not be operating properly? As stated above, everywhere it should be wet, it is.

A retired engineer in the family analyzed all the specs (for fun:-)) for the heater, A/C and humidifier when we built the house. (he knew the formulas and used a slide-rule.) He assessed that the units are at max operating capacity for the size of the house. He estimated that during extreme temps (winter or summer) there would be particular pockets of the house that would be less than ideal. He was very accurate. Overall, the units do there job.

My issue is not with the heat flow in winter. the variable (multi- ?) speed unit operates at a low speed when house is balanced. the humidity level seems to have dropped. It can be felt which is what caused the measurement devices to come out.

What should I look to clean, adjust, test, replace? I am thinking the focus is on the humidifier unit, but maybe I am missing something. Your thoughts and suggestions appreciated. TIA