Do you have any frost on the lines going to the outside unit. If the large line is frosted your evaporator (Cooling coil in the furnace) could be frozen up. Operating it with the fan in automatic with the cooling on will sometimes be the cause of this condition.

If there is not frost on the large line going to the outside unit and you are getting air circulation inside the house the problem could be a loss of refrigerant charge. Look at the joints where the pipes were soldered to the inside and outside units for signs of oil. If you find oil at any point it is the indication of a refrigerant leak.