As to the above points, I kept his substrate really moist (Not wet, just damp) and I think it's likely that the humidity was higher closer to the soil than the sensor could reach. The shed was stuck on his legs to start with - I actually managed to get a bit of it off his sides when I brought him home and soaked him, because I was also worried he was dehydrated.

Otherwise I'll keep your comments in mind in the future! Someday I'll try again.