Samsung Freezer not cold

Model Number: RF267AERS
Brand: Brand
Age: 1-5 years

The freezer is no longer cold. Refrigerator temperature is good however. I even defrosted the whole thing, removed back panel and checked for ice on coils in both the fridge and freezer. There was ice build up in drain in fridge and I cleaned that all up. But still, the freezer is not getting cold. No ice on freezer coils and defrost system is still working there (could feel it heat up when I took panel off) Could it be the sensor in freezer? Fan in freezer? Something in the back panel? Any ideas about what to look into next would be so appreciated!