Automatic defrosting systems consist of a heater, a thermostat and a timer that controls the defrost cycles. According to Appliance411, the timers in the defroster fail fairly frequently and need to be reset or replaced. The result depends on when the timer stops working: If the timer stops working during normal operation, then the refrigerator is never defrosted and ice builds up along the walls of the freezer. If it stops working during a defrost cycle, the freezer will fail to return to cooling.
Defrost systems also contain a thermostat that could fail. This thermostat is responsible for turning off the heating element when an appropriate defrost temperature is reached. If the thermostat fails, even if the timer is signaling the beginning of a defrost phase, the thermostat is failing to turn on the heater and the temperature of the freezer never rises enough for the frost to melt. Another possible symptom of a broken thermostat is a wild variation of temperatures within the freezer. The thermostat should ensure that the freezer heats just enough to thaw any accumulated ice but not enough to thaw the freezer's contents.
Finally, the heater itself could be broken. If the heater fails, the timer and thermostat are both signaling that it is time to heat the refrigerator a bit, but the heater fails to respond and the defrost cycle ends without any defrosting having occurred.
Since the cause of the problem could be any number of parts in the defrost system, problems are best handled by a trained appliance technician. Consult the manufacturer, as the problem may be covered by your warranty.