if its 134a 2-3 lbs on the low side should give you an evaporator temp of around 0 degrees which is actually a little high but close. If it were low on refrigerant low side would be in a vacuum , sounds like you have an airflow problem. If the evaporator is completely blocked up with ice you wont get the airflow you need. The defrost thermostat, heater and timer are common failure points. An easy test would be to unplug unit and let it defrost and if it works good after a day or so with the doors left open then its not defrosting automatically. Or you can remove the cover on the evaporator and have a look