Look in the rear of the freezer (the rear inside wall panel). Do you see a heavy snow-like frost build-up, or, is it bulging out from frost? Do you see any snowy blockage in the damper? If so, you have a defrost problem. If no heavy frost build-up is present, the problem is probably with the damper. It may be blocked physically by the arrangement of food items, or possibly malfunctioning.