Probably the defroster timer. It can be tested.
start by taking everything out and making sure there’s no ice build up behind the back panels and that they can “breathe”.
I would also pull the whole unit away from the wall and shop vac the entire back, with the rear exterior panel off.
check for recalls on your model, they’re surprisingly common.
Check your door gaskets. Check for frost build-up. It would be worth the price of a service call to ensure safe food and reasonable power consumption. I had a service call for mine, and he vacuumed the coils, checked the refrigerant and the gaskets in a short period of time. Some things require some skills. He also recommended how often to clean the coils, and what to watch for.
Here are some ideas on why it may not be working: https://www.repairclinic.com/RepairHelp/How-To-Fix-A-Freezer/96---/Freezer-Not-Freezing
There's a small drain hole in frost free freezers that gets clogged with ice. This will prevent it from having a proper refrigeration cycle and then it can't freeze properly. This should be fixed by turning it off and thoroughly defrosting the unit. Do not use anything sharp on the ice as refrigerant coils are situated within the freezer compartment.
I had that problem with my Fridgidaire frost free upright freezer years ago. Thought it was a goner (the food had to be thrown out). Serviceman thought it was a goner. Then he realized there was an interior back panel. He took out all the shelves, got that panel off, and found a complete sheet of ice. Melted the ice, put it back together, and it's been fine ever since. Hope this helps.
Do you have food stacked in front of the blower that cools the freezer. Cold air has to have room to come into the freezer and circulate. I f you block the vent there are areas that will not freeze.