Trouble with humidity

[Malum Argenteum]

I've been keeping his humidity around 55-62% pretty consistently since he was a baby, now for the past couple months it's suddenly stuck in the 40s, I'm not sure what I'm doing wrong.

Sometimes that's caused by a malfunctioning meter (which is very common -- they're pretty easy to wreck with water contact, or dirt/dust clogging). I'd recommend getting two more quality digital meters (one to check the possibly broken one, and a second to adjudicate a disagreement).