As you are currently using, a bowl with saturated paper towels, aspen shavings, or spagnum moss, setting over the under cage heat source, usually works quite well to keep the humidity up. However, as stated in the previous response, if the cage has a screen top covered by a towel, you are probably losing a fair amount.
Another possibility is that your humidity gauge is not functioning properly. A way to check any humidity gauge is to saturate a handful of paper towels and place them in a clear gallon size plastic bag with about half an inch of room temp. water standing in the bottom after the saturated towels are in the bag. Place the whole humidity gauge or probe, depending on type, in the bag and seal it with a twist tie. Keep the gauge above the water and the saturated paper towels. Wait about 10 minutes and check the gauge reading. It should be reading between 95 and 100% relative humidity. If not, adjust appropiately if it has an adjustment screw. If it is reading 5 or 10% low, and you can't adjust, you can just take this into account and add that much to any reading. If off by much more than 10% to 15%, I would look at replacing the gauge.
Kelly