The damper control could be the problem. Look inside it and see if it is frosted up or closed. If frost is forming inside the damper it is because it isn't closing completely and needs to be replaced. If the damper is closed replace it. If the damper is open and has no frost, these are instructions for defrosting the evaporator coil.
Usually with this symptom the evaporator coil at the rear of the freezer is frosted over and air is not being circulated through the coil and into the freezer. To confirm this look at the lower back of the freezer and see if there is frost built up.
Begin by unplugging the refrigerator from the wall.
If there is frost remove the food from the freezer. Remove the icemaker by removing the 2 screws above the ice mold and pull the icemaker away from the wall. Unplug the icemaker from the wall and remove it.
Remove the screws in the back cover and remove it.
Defrost the evaporator coil with a hair dryer.
Do not use a sharp object to remove the frost and ice. The evaporator coil is aluminum and easily punctured.
This will make the refrigerator work 5-7 days and the coil will frost over again. The defrost system consists of the defrost timer, defrost heater, and the defrost termination thermostat.
You can test this system after the freezer has reached temperature by turning the shaft in the defrost timer clockwise slowly until the compressor and inside fan turn off. Wait 5-10 minutes and look at the bottom of the evaporator color. If the heater is glowing the defrost timer is the problem.
To access the defrost timer shaft remove the plastic plug to the left of the thermostat knob. If the heater doesn't come on the problem will be the defrost heater or the defrost thermostat. You can check them with an ohm meter. They are both on the evaporator coil.