If it frosts up, it is freezing.
Unless this is a newer frost free unit, you probably just have a poor lid seal.
Check the fit visually
Also check the rubber gasket for cracks etc.
One way is put a strong flashlight in the unit facing up, turn it on, turn the lights off in the room and check for light leaks.
Take a peice of paper and slip it between the freezer body and the lid. Try it all the way around the seal.
If it is loose you have an air leak. | 
Chest freezer frosts up instead of freezing 
