Defrost timers frequently fail and stop keeping time. This allows excessive frost to accumulate reducing air flow, eventually stopping it altogether as it never switches into defrost mode. The defrost heater can burn out and open the circuit in the defrost system. Even if the timer advances into the defrost cycle, the heater will not produce any heat to defrost the evaporator coil. The timer goes marrily on its way back into the cooling cycle effectively accomplishing nothing.
More frost builds up eventually plugging the cooling coil totally.
The defrost termination thermostat opens the circuit to the heater when it senses a set temperature. This temperature is usually high enough that the cooling coil has been totally defrosted by then. If this device fails and remains open permanently, the heater never gets power to defrost the evaporator. If the defrost termination thermostat contacts short out, it will allow the defrost heater to stay on as long as the timer is in the defrost mode. A symptom of that is usually greatly fluctuating temperatures in the freezer compartment possibly causing thawing and re-freezing of food stuffs.