Why does the low pressure A C line turn white and very cold but the inside of the car does not get cold?

Why does the low pressure A C line turn white and very cold but the inside of the car does not get cold?
Make sure the heater is not on, sometimes the door in the heater/ac box is not working properly or the valve controlloing the hot water to the heater is not stopping the flow. An ac will not out perform a heater. The last thing it could be is that the evaporator is freezing up, or there is a clog in the line somewhere.