Venta humidifier is not humidifying much.

I have had my Venta 207 humidifier for two years but it has been humidifying very little recently. It runs, and there is warm water on where it should be, but it now takes about five days to empty the tank when it used to take one day. I used to run it at 2 on warm mist, but now moving up to 3 still doesn't help. I know the air in the room is quite dry. I brushed the ********, and even tried vinegar, but nothing seems to help. How can I speed up the output?