Hello Wendy,
You should first check to see if there is any ice build-up in the floor of the freezer compartment. It may be hidden under a false bottom. There should be screws to remove it. You should also check underneath the unit and make sure there is a pan under there. There is a drain tube that should be suspended above the pan. This is how the defrost water is disposed of, (it goes in the pan and the condenser fan blows warm air across the water, evaporating the water back into the room).

Hello Wendy,
This unit may have a rotary compressor. There's a good chance the compressor is going and the lower part of the evaporator is the only frost pattern you have. It may have an ice ball formation. If so, it may be a good time to shop for a new unit. This was just an after thought after discussing it with some other experienced technicians.