Your defrost system needs addressing.
Their are 3 parts to the system.

Timer

Heater

Terminator

 

Normal operation:
The timer will rotate around for about 8 hours or more and then it will kick into defrost for 21 minutes. The heater will come on and get very hot. Hot enough to cook anything in the freezer in 21 minutes including the kitchen. Thank goodness it has a Terminator thermostat which shuts down the heater when it reaches a temp which melts that frost. So what can go wrong? The timer motor can die or the timer can simply hang up in the defrost mode. The heater can blow out just like an ordinary light bulb over time. The terminator will actually swell up and fail also. If your model is not a self defroster then I would say your door seals are leaking more air than they should causing more ice buildup from humidity seeping into the ice box.

 

About the pictures:

The timer is a typical mechanically operated one with a feature to manually advance it with flat blade screwdriver.

The heater shown is prewired with quick plug and defrost terminator. Very easy to replace!

The terminator on most of these ice box's will only have continuity if it has been sitting in a freezer at 25°F for a cupla hours if you want to test it you must do it quickly. Its job is to open the minute it reach's 50°F. It kills that heater so the defrosting can carry on for 21 minutes without the possibility of causing a fire.