Check the evaporator coil for frost buildup on the back. Check for frost on the suction line, the biggest line returning to the compressor. If frost is evident at these places you have a defrost system problem. These systems have a defrost timer that defrosts the coil at preset times. Check the timer for proper operation by marking the position of the dial and check back to see if it changes. If it does not change position the timer motor is most likely bad. If the dial is not rotating automatically you can manually turn the dial in the direction of the arrow and put it through a defrost cycle.