I think I was having similar problems with my humidor, when it appeared to be holding a good Rh, but then it would drop. My problem ended up being improper seasoning. I was trying to do it too quickly by wetting the wood but immediately putting my cigars in there.
I ended up taking the long, slow route:
1) Took all my cigars out and placed them in a sealable plastic baggie
2) Wetted down some paper towels with distilled water (didn't have a clean sponge)
3) Wiped down the entire interior of my humidor until the wood looked wet. Note: it wasn't soaked where water would pool or drop. It was just wet so that the wood changed color to indicate moisture was added to it. This immediately created a humid environment of over 90%
4) Put in my humidifier and a shot glass filled with distilled water.........Waited 24 hours. This dropped the humidity down to about 73%
5) Did the same process all over AGAIN! That is, I wiped the wood again with distilled water and waited ANOTHER 24 hours! This was the longest wait of my life but I was thankful to do things the right way!
6) At the end of the waiting period, the humidity was around 70% - 75% with no cigars in there. I filled that sucker up, and at first, the humidy spiked around 85%, so I opened the humidor occassionally to help balance things out, as well as close my humidifier (credo). Now it's around 69% and holding nicely.
So if you used the painter's tape and you still have that low of a humidity, I would say that the interior of your humidor isn't seasoned properly. You want to make sure that the wood has absorbed all the natural humidy that you create so that it maintains that level when full. I knew that my humidor was properly "charged" when it was able to maintain the humidity I wanted WITHOUT any cigars in there for at least 24 hours.