I have attached a copy of the tech sheet for your refrigerator. It has a great troubleshooting guide and test for the defrost system.
Your refrigerators defrost system has only three parts to it.
1. Defrost Heater: Part # AP4078863
2. Defrost Thermostat / Bi Metal: Part # AP4310486
3. Defrost Timer / Electronic Control Board: Part # AP4366538
My personal experience with this type of refrigerator with the control board for the defrost system is most often the problem is with the control board.
Sometimes I find a bad defrost thermostat. Most of the time the heater is fine.
Let us know what you find or if you have any other questions.