some things to try
Did you condition your humidor when it was new? This involves wiping down all inside surfaces of the humi with distilled water to get the spanish cedar lining up to operational humidity. Even if you did this once, this may be worth doing again if you are having trouble. Be sure you store your cigars elsewhere for at least 24 hours after wiping down the inside, because the sudden humidity jump can cause them to burst.
If this was never done when the humi was new, the cedar lining (which is kiln dried) will soak up moisture from the air until an equilibrium is reached, and it can be hard to get to 70% for awhile.
Also, if it's a top loader, check to be sure there are no major gaps at the corners of the lip hanging from the lid (which should fit snugly inside the walls of the lower part when closed). If there are gaps these little channels can allow the humi to breath too much. These gaps at the corners can be filled with shims made from the cedar wrappers around some cigars like Fuente Chateau,etc. Just cut shims with an exacto knife, fill the gaps and trim for appearance, then swab the corner with distilled water so the shim gets moist and expands to fill the gap.
Hope it works out for you