I have corrected your model number information for this topic, thanks!
I looked up your model number and your refrigerator defrost system only consist of three parts.
1. Defrost Thermostat, Part # 10442411
2. Defrost Heater, Part # 10428409
3. Defrost Timer, Part # R0131577
As jumptrout51 said I would suspect your defrost heater or the defrost thermostat.
I would check them with a ohm meter, but if you don't have a meter you can bypass the defrost thermostat by jumping it out. Then manually advance your defrost timer to the defrost cycle. Then wait and see if the defrost heater gets hot.
If the defrost heater gets hot then the heater is fine and you will need to replace the defrost thermostat.
If your defrost heater does not get hot with the defrost thermostat jumped out and the defrost timer advanced to the defrost cycle then chances are the defrost heater is bad and need to be replaced.Warning:
Only jump out or bypass the defrost thermostat for testing purposes. Do not leave the defrost thermostat jumped out.
If you do it could cause a fire!
You may also be able to tell if the defrost thermostat is bad by just looking at it.
In the photo below the defrost thermostat on the left is bad. Notice how the top is popped up and out of place. This is just how I sometimes find them in the freezers.