Are the compressor and fans on all the time except when the unit goes through a defrost cycle.
If yes then remove the cover in the freezer and check the evaporator coil.
If it is frozen over with frost/ice to the point where air cannot be pulled through it you have a defrost problem.
If there is just frost in one area usually where the freon enters the coils then it is a sealed system problem.
Note that sealed system problems usually are not worth fixing as you need a pro and it will not be cheap.