Heavy frost buildup in a freezer — on the walls, on the food, on the coil behind the back panel — is almost always one of three things.
First, a failed defrost system. Modern freezers run a defrost cycle every six to twelve hours to melt frost off the evaporator coil. The cycle involves a defrost heater, a defrost thermostat, and a defrost timer or control-board routine. When any of the three fails, frost builds up until the coil is encased in ice and airflow stops. Pulling the back panel inside the freezer and finding a solid block of ice confirms the diagnosis. Typical repair is $250 to $450.
Second, a torn or compressed door gasket. A gasket that doesn't seal lets humid room air into the freezer; that humidity freezes on the first cold surface it hits, typically the ice-maker, the walls near the door, or packaged food. Inspect the gasket for cracks, tears, or places where the magnet has weakened. Replacement gaskets are $50 to $150 and easy to install.
Third, a clogged defrost drain. The defrost cycle produces water that needs to drain into a pan under the unit. If that drain tube freezes or clogs with food debris, water pools at the bottom of the freezer and refreezes into a sheet of ice on the floor. Clear the drain, steam or warm water from the inside, and the problem usually stays fixed.
One more worth checking before service: over-packing. A freezer packed so full that air can't circulate will frost unevenly. Leave a few inches of clearance around vents and pull food away from the back wall.