They don't leak down all that often.
Before you assume it needs refrigerant:
Are the Coils clean? The Condenser coil on the rear of the unit is easy. The Evaporator is under a sheet metal cover on the front of the unit.
Is the Filter clean?
If it's Ducted, are you sure there isn't a problem in the ducting? Likely where the A/C is installed.
Are you running it on High Fan, reducing the chance of the Evaporator Coil frosting over? If your unit is Ducted, a Freeze Sensor will detect that and stop the compressor from further cooling till it's satisfied the coil isn't freezing. If it's not ducted, freeze over will reduce, probably stop, cooling.