Here are a few reasons why this problem may occur:
1) The air circulation is very high (air is fully exchanged several times per day – the humidifier needs to starts its work from scratch)
2) The insulation of your home is not adequate or has open space with high ceilings, which takes the humidifier much longer to increase the humidity level.
3) If there is a big difference between outside temperature and inside temperature – the colder the air the less moisture it is capable of holding, and the more the air is heated the lower the percentage of relative humidity (the amount of water vapor in the air, compared to the amount the air could hold if it was totally saturated).
4) The position of the unit – if it is placed close to a door, window or air vent, it can cause an increase of air circulation.