Remove the cover in the freezer so you can inspect the evaporator coils.
Be sure that the compressor has run for at leeast half an hpour. If you check the coils during or shortly after a defrost cycle you may come to the wrong conclusion.

If the coils have no frost or just frost in one area then it is a sealed system problem.

This will be an expensive repair. Often it makes more sense to put the dollars towards a new unit.