It could be a number of things. Problem with the fan motor, or fan motor controls, or even the thermostat. It could be loss of refrigerant causing the unit to not have the capacity to cool as it needs to. A reduction in cooling capacity may make it seam like the air you feel coming out of the duct is less. With the possibility of a few problems you may want to call a mechanic if you do not feel you can tackle the problem. Some trouble shooting is required.