Is the compressor motor running continually trying to drive the freezer temperature down to 0°F or does it stop occasionally for a short time as if it seems as though the correct temp has been achieved, even though it hasn't before starting again?
First check that the freezer door seals are OK and that they're providing a good seal and not letting any cold air escape from the compartment.
To do this, place a sheet of paper between the freezer door and the door jamb, and with the door closed normally i.e. no pressure on it by you to keep it closed, try pulling the paper out. It will come out with a bit of an effort but not easily and it should definitely not fall out by itself. Do this at various points around the door, top, bottom and both sides. Also make sure that the door is straight and parallel to the cabinet in case of a hinge problem that is preventing a good seal.
If the seals and alignment are OK perhaps there is a problem with the freezer temperature thermistor (supplier example only)
If you decide to replace the thermistor, here's a video that may help. Search online for WP2188820 to find suppliers that suit you best.