Here are a couple of thoughts
If you have another humidor with a hygrometer you feel is working try swapping them out. Put the reliable one in the new humi and the new one in the old and then compare. Who knows you might have a defective hygrometer
Another suggestion, and this might make a few people cringe, is to take a very high grit numbered sandpaper, like 150 to 300 and very lightly sand the cedar, then using a lint free cloth wipe out the inside. this will possibly open up the pours of the wood and allow for better absorption.