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When you say it is running, is the compressor running?
You can usually hear it but should also be able to feel it vibrating.

If running is it on all the time (other than when it is in a defrost cycle) or is it cycling on/off every minute or so?

If on all the time remove the cover in the freezer so you can inspect the evaporator coils.

Is the evaporator/freezer fan on when the compressor is on?

If yes then:

If the coils are heavily ice/frosted over then you have a defrost problem.

If there is no frost or just frost in one area (usually where the freon enters the coils) then it is a sealed system problem.