The most common question:  "I can't keep the relative humidity on the inside of my humidor at or near 70%...

First, check the solidity of your box. Are the seams and joints secure and closed.  If it has drawers, or if your box has a glass insert in the top or sides, you are fighting a tough battle. If not well made, then all of that has to be carefully sealed with some silicon (or something) on the outside of the box. If the top opens up, check the seal of the lid when it closes against the box. Should be a slight whoosh sound when you close it to demonstrate that it is sealed.  Your box has to "breathe" but it has to maintain the proper environment for your precious smokes. 

If everything is OK structurally, then you might need to "season" the interior of your box to make sure that its not the box itself that is leaching your moisture.  There are several different methods and materials to guide you to a proper season.  I take an empty humidor box and moisten the interior walls and bottom and lid with distilled water from a well squeezed out damp paper towel or wash cloth, then set an open dish of distilled water inside until the next day when you wet down again. Repeated wettings and open water will sooner or later get the wood of the box up to the proper humidity level. To eventually test it, the day after a wetting, don't put the dish of water in there, but put your best and most accurate hydrometer in there and see where it levels off. Put you oasis of choice in there and wait 2 or 3 more days more to make sure that the RH is maintained. There are chemicals and things said to aid in this process, but I have not tried them, so can make no recommendations.  This is a slow process, but it is vital to start your cigar storage off on the right foot, or you will fight it long and hard down the road, and your smokes will eventually suffer.

Humidors:  Seasoning