Verify Humidifier is Working
You can also get a hygrometer (reads out humidity level) and see if it increases as your heating system runs. Even if the humidity number stays constant as temp in the house increases that is an indication that you are adding moisture to the air.
With the humidifier on my gas furnace I usually see a couple % increase in humidity along with a 1 or 2 degree temp rise for a typical heating cycle. But with the cooler supply temps of your heat pump I wouldn't expect that much. As BT mentioned connecting your humidifier to the hot water will help.
Live long and prosper.