Analog hygrometers and thermometers are notorious for giving false readings. They can be off by 10-20%. This would make sense if your humidity is actually at 40% and being read at 25%. Almost every experienced keeper here would suggest you switch to a trusted digital brand.
AcuRite is one that is accurate, widely used and fits into most budgets.
Glass tank + heat lamp will also zap the humidity out of the tank, but sometimes that's just what happens. If you haven't already, you can try covering the screen top with tin foil, plexiglass, or a damp towel to trap humidity, and cut a slot for the lamp.
Misting is a great way to up the humidity, but ideally you don't want the enclosure to be wet all the time. An enclosure that's wet 24/7 can cause scale rot and other problems.
Seems like you're doing good with the right substrate. That stuff holds humidity well.
My suggestion is get a digital hygrometer/thermometer and if it still is showing up low then modify the tank further to hold in humidity or switch to a tub (which holds humidity extremely well compared to tanks).
Good luck!
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