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I have a three houses in COLD northern Illinois with whole house humidifiers.  I maintain them myself and no problem in two of them.  But, just moved into 2500 sf home and first heating season. 
It has a York diamond 90 furnace and aprilaire 700 auto humidifier.  I replaced a faulty transformer and the humidistat series 56 last week.  Runs as expected and water pad is wet and water runs down into tube steadily now.  But, humidity never goes above 26% even when I use manual mode on humidistat and set it to max (7) on controller which should be 45% RH. Measured at Honeywell Wi-Fi thermostat and separate hygrometer that are only 2% different.  I can't find any leaks in ductwork in attic and no condensation on windows and it's 5 - 10 degrees F right now. any advice appreciated.

Is is powered constantly so that when the humidistat turns on the humidifier will run?  Occasionally they are wired with the heating circuit and only run when the furnace is heating, and sometimes the humidifier is not able to run long enough in this scenario.  Usually the automatic models require constant power.  Is the furnace fan running anytime the humidifier is on?

Do you have an air exchanger running in the house?  If the outdoor humidity is only 30% in Chicago it could decrease the indoor humidity if it's left running too long. 

There may be a duct that brings in fresh air to the gas appliances or it may be piped directly into the return air duct.  I have seen older homes upgrade their equipment to direct vent and no longer require the outdoor air supply, but no one removed it, so it continues to bring in outdoor air. 

Or if you have a wood burning fireplace, ensure the flue damper is closed, when not in use.

That model of humidifier should satisfy a 2500sq ft home.  Some people will use a copper water line and a hot water supply, instead of cold water, to increase the humidity output of the humidifier.

Thanks for responding. The humidifier has a separate outlet so it has power and furnace fan runs whenever humidifier is on.  Home built in 2005 so I don't think it ever had an outside supply.  Outdoor humidity is 60% and I don't have an air exchanger or a wood fireplace.  Hot Piped from water heater and copper supply line. I'm at a loss.