Some frost inside the freezer is normal, because each time you open the freezer door or lid, warm, moist air enters the freezer and forms frost. But if excessive frost forms quickly inside the freezer, it could be that a damaged lid or door seal is constantly allowing warm, moist air into the cabinet, resulting in excessive frost. Check the door or lid seal and replace the seal if damaged.
A crack in the plastic liner inside the freezer cabinet can also allow warm air into the freezer and cause frost. Check the inner liner and repair any cracks using a liner repair kit.
If you see frost on the evaporator cover in the back of the freezer and the freezer isn't cooling well, then a failure in the automatic defrost system could be causing the frost. If the defrost heating element that melts frost off the evaporator fins fails, frost will cake up on the evaporator and interfere with airflow and cooling in the freezer. A failed defrost bi-metal won't allow current to reach the heater so a failed defrost bi-metal will also cause frost on the evaporator and inhibit cooling in the freezer. A failed defrost timer won't advance into the automatic defrost mode so the defrost heating element would never turn on to melt frost off the evaporator. Check the defrost system if you see frost only on the evaporator cover.