Freezer is completely covered with white ice. The freezer is not defrosting, but is staying cold.

Question

The freezer is not defrosting, but is staying cold. The refrigerator is warm. We have replaced the automatic defrost thermostat (round metal object with pink and orange wires). We are having to manually defrost it every week, it seems. It is a GE Hotpoint Model HSS251FMD WW and Serial # is GD283O87.