i am having the same problem right now and tried everything! ...today i placed a wet sponge in the hot spot and humidity jumped from %43 to %72!! i dont know how long it will last though. also today i am going out to get a small live plant to place in the cage ..that should make humidity ramian high without a sponge but im not %100 sure if it will work.

~KP
p.s. good luck!