It could be that the hygrometer you have is not accurate. What kind of heating do you use? Under the tank or a light, maybe a CHE? If its any of those do you have a thermostat? I ask because his heat source should be on during the day too, with a thermostat to regulate the temp. Are his scales dull looking, or has he gone in blue? Those help to indicate that he could be going into shed. I would also check his underbelly, not just the enclosure, for mites. They hide between the scales.