If you have an air conditioner or heat pump that seems to be cooling well but doesn't seem to be removing humidity, you have a search on your hands. There are a few reasons why a heat pump or AC system might not dehumidify when it's supposed to, and to find the reason, you have to start inspecting several parts of your house and the system. Don't fret, though; you will eventually be able to find the cause.
Inside and Outside Leaks
One potential reason for the increased humidity is a leak, either inside or outside the home. You could have a leak in the hot-water lines, which would add humidity as the water traveled outside the pipes. You could also have poor insulation that allows humidity from outside to leak in. If the weather in your area has been a lot more humid lately, then air leaks are something you're going to want to search for.
Faulty or Poorly Sized Systems
Another common problem is that the system you have might be poorly sized. If it's a system you've used for a few years and have not had a problem with before, then sizing is likely not the reason for the increased humidity. But if it's a new system, then you have to look at sizing. A system that's too small for the square footage of your home won't be able to move all of the air around and through the system adequately, resulting in increased humidity because it just won't have the power to remove it all. A system that's too large or too powerful for the home will cool too quickly, meaning the air won't have a chance to pass through the system completely, either. Again, that would result in excess humidity in the air.
Flooded Slab Ductwork
If your house is on a slab and has ductwork running through the slab, you might want to look at flooding issues. These can be from a slab leak or from flooding and rain in your area. The moisture in the ducts can gradually work its way up into your house. You have to have the water removed by professionals to fix that.
Food, of All Things
One more potential cause is cooking, of all things. If you've been cooking a lot more lately, especially things like soups, chilis, and stews, and you haven't been opening a window or turning on a range fan to remove the steam and fumes, you could end up with more humidity in the air from when the moisture in the food evaporated. Start using your range fan more or open a window to fix that problem.
If none of these issues seems to be the culprit, or if the system is sized poorly, call in a heat pump or AC repair person and have the system fixed. Don't let excess moisture stay in your home for too long.