Yesterday, I noticed that my Kenmore Coldspot Freezer while cold, was clearly not cold enough as the ice cream wasn't frozen and other food was cold but not frozen. I put a home thermostat in there to try to check the temp (not sure that it would be the most reliable, but thought it'd give me some idea anyway), and it read between 10-16 degrees, but never any colder.

Can anyone please help me out with steps to diagnose? The refrigerator seems to be operating fine....