It's extremely dry where I live and spraying doesn't do much. My house's humidity is usually around 25%. I used to have a BP and I tried the wet towel thing, spraying with water, moving him away from the window, but it was always around 50%.
Did you try switching to a tub? In the winter my house's RH is 10-15% but I had no problem maintaining 80-90% humidity in my BRB tubs.