Aprilaire 600 humidity level too low

you may try running your blower 100% of the time.this should allow the humidifier to stay on even when your not heating. once the humidity level gets to where you want it, you may be able to put the fan selector back to auto. also you mentioned you have a variable blower. this is great if it is set up to run at approx. 20% fan speed when it is not heating or cooling. at 20% you can circulate enough air through the house to humidify without being able to hear the noise of the blower. it will really increase the comfort level of the house if you tend to have hot and cool spots. if it was not set up this way, contact the installer and ask if they can do this for you.