Auto defrost works by a low-powered heating element being switched on briefly by the thermostat at the end of each cooling cycle and melting the ice formed over the area of the evaporator. The water is then channeled and drains into a bowl mounted on the compressor where the heat from the hard-working electric motor and compressor vaporises it.

Assuming no wiring faults your problem is likely caused by either a faulty thermostat or a faulty heater.

There are 3 components to a defrost system. The heater, thermostat, and timer or control board One or more of those components are most likely bad.