Hi,
There can be a couple of things that could be happening...
The first and easiest thing is to check the air filter. Make sure that it is clean... clean it or replace it. Next check the air vents to make sure that they are open and giving you good air flow...
Also check to make sure that you are not icing up on the indoor coil... ice will tell you that you are low on refrigerant. You also could be low and not be icing over, but you would need to have gauges on the unit to check that.
Finally check and clean the outdoor coil...this also can give you problems if the heat is not being removed from the outdoor unit
efficiently...
You may also have a unit that is sized in such a way that it hits the balance point at about 85...that means that your heat gain is equal to the capacity of the unit...I often try to size them for about that here in Northern PA. because wqhen seldom get hotter then that and the unit then is much more effective at removeing humidity at lower temperatures...
You can also check out the tips that I have wrote about
Air conditioning trouble shooting and maintenance...
Here is one about cleaning the condenser coil and at the bottom of that one are links to others..
Condenser Coil Cleaning Condenser Coil Cleaner
http://www.fixya.com/support/r3850183-condenser_coil_cleaning_condenser_coilheatman101