Empty your humidor, and put an energized flashlight in there, turn off all the lights, this will help tell you if there is an obvious leak.

I usa an Oasis in my coolidor, and seconds after I open the lid, the reading usually drops quite a bit, but it recovers very quickly when closed. I can hear the fan run for less than a minute.

I have never calibrated the oasis, since I really can't think of a procedure to do this with the fan involved, so I keep a cheapo wal-mart hygro that I have salt tested just to verify the oasis, mine reads about 3% higher than actual.

you could re-season by moistening the lining slightly with distilled water, and let the oasis run for a day or two, or do like I did. get yourself the biggest frikin cooler you can afford/house, go to heartfeltindustries.com and pick up a pound or more of their Rh beads, (or stick with the Oasis), then use the desktop himi as a "dry-box", ( I use mine for cigars that draw to tight, a day or two in solitary confinement usually adjusts their attitude sufficiently)