Freezer not getting cold enough. This is a Kenmore chest freezer, less than 2 years old. The

Question

This is a Kenmore chest freezer, less than 2 years old. The compressor runs continuously, but it only gets down to the mid 50s inside.
The front wall inside is pretty cold, and there is some frost accumulation at the top front in a kind of band along the top inside surface. The other walls, back and sides, don't feel so cold internally, just cool. None of the external walls are particularly hot, although just above and around the compressor is kinda warm.
This started a few weeks ago. For a while it would get down into the 40s. I turned it off yesterday to be sure there wasn't ice builup somewhere, and now it won't even reach 50!