The 2 most common part(s) or condition(s) which cause the symptom Refrigerator not defrosting are listed below starting with the most likely. Check or test each item and watch any available videos.
RepairClinic Item # 1399613
If the refrigerator is not defrosting the defrost heater assembly might be burned out. Several times a day the defrost heater assembly turns on for a few minutes to melt away any frost that may have accumulated on the freezer or refrigerator evaporator coils. If the defrost heater assembly has burned out, the frost will build up on the coils and eventually no air will be able to travel through the coils to provide cooling. The defrost heater can be checked for continuity. If it doesn't have continuity it should be replaced.
RepairClinic Item # 1531075
If the refrigerator is not defrosting, the main control board might be defective. A few times a day the control board is supposed to turn off the cooling system and turn on the defrost heater to melt away any frost that may have accumulated on the freezer evaporator coils. If the control board is defective it could cause the system to never cycle into defrost.