Excess frost and ice signifies a problem with:
The door seal is damaged or misaligned, causing cold air to leak out. Replace the worn seal.
The defrosting system is malfunctioning. Check components such as the defrost timer, heater and end thermostat.
Clogged drain line, preventing defrost water from draining out. Clear the drain line of any debris.
Excessive humidity and humidity in the environment. Dehumidify the room and improve ventilation.