Is there a light on that is providing heat? In the daytime, a lamp that puts off heat will dry things out quickly. If you can get away with a compact florescent bulb it may dry out less. This will depend on the how your keeping your temps up.

Live plants such as bromeliads also help keep humidity up.

I usually have to mist twice daily and the lowest mine gets is about 50 percent.
You may also want to compare your humidity gauge to another one since they are not usually the most accurate things.

Pics of the setup may help someone provide other ideas too,