Sounds like there's an defrost problem.
This could be due to either the refrigerator not going into defrost mode (control board?), a faulty defrost heater, faulty defrost thermostat or perhaps the drain leading from below the evaporator unit in the freezer compartment to the evaporator pan under the compartments near the compressor motor is blocked, preventing the defrost meltwater from draining therefore the ice builds up again when the defrost cycle is finished.
With most refrigerators the defrost cycle occurs once every 8-10 hours and lasts for about 20-25 minutes.
During this time the compressor and evaporator fan are both turned off and the defrost heater is turned on to help speed up the defrost of the ice covering the evaporator unit. When the temp in the freezer near to the defrost thermostat reaches the preset operating temp of the defrost thermostat (~46°F), it operates and turns the heater off to prevent damage to the evaporator unit.
Check if the refrigerator enters the defrost mode and that the heater is turned on.
Here's the service and wiring sheet that will help. Here also is a video that shows how to test if the bi-metal defrost thermostat is OK . If it is faulty it will prevent the heater from operating.