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Sounds like it’s doing what it’s supposed to do. The initial starting will only go to zero F or slightly less, and as time goes on it will get to a lower setting as the load comes down. Just so you know, longer run times are desirable in a freezer for electrical efficiency. Product in a freezer should never come above -5F for long term storage.   But it you want to change it, what model do you have and what controller does it have. If it is electronic, is it properly programed and locked? What is the controller set at? At what temperature do you see the compressor light go off on the controller? Does the compressor continue to run after the cooling light goes out?