Do you have any sources for humidity in the house?
i.e. Hot tub, water fall (artwork, decor, etc), large aquariums, clothes drier vented properly (vent disconnected behind drier may be more noticeably since the laundry room will get hot. Sometimes they are not as noticable though. Sometimes you will see an unusual accumulation of stuff get on you air filter for your air conditioner. Compare the color of what you find on your air filter with the color of the lint you get out of your drier filter. A drier vent venting under the house is not good either.
Not only do you add all the moisture from your wet clothes to under the house... but the heat rises up against your floor and cunducts the heat back into the house. Turn your drier on and check the vent outside. If its blowing alot of air, then chances are its not come undone. If you can get under your house *SAFELY* then a visual inspection is best.
Factors that contribute to an air cond system to not remove humidity properly include...
High Air Flow
Just a side note:
I've seen systems with the new refrigerant (R-410 / Puron)
appear to be fully charged by the gauges, but an actual
capacity check revealed it was not right.
(For the tech's out there... 19 delta T Dry bulb 8 delta T Wet Bulb... removing plenty Sensible but not latent. This is on a Trane XL14i with the correct air flow.
No duct leaks. Capacity check revealed about 19000 btu
total but should have been closer to 38000 at the time.
Don't have all the info available right now to show you... just be warned the R410 might fool you even though the charge looks right.)
Other factors to consider...
Cheap hygrometers from wal-mart are just that... cheap.
Get you hvac guy come in and test with an accurate meter.
Does the house feel like a cave? Cold, damp, and clammy?
Is the hygrometer located near a room with sources of water? i.e. Kitchen, bathroom, laundry.
It should be located on an interior wall also.