One thing about the small compressor coolers is they get clogged with dust bunnies, hair, etc. it doesn't take much for them to stop working properly. The thing I and others I know have experienced with them is the condenser gets clogged, and compressor and condenser will then heat soak the cooler. This will cause a condition where everything on top is frozen but everyone below is thaw.

I like to add insulation and reflective foil to such coolers. On the outside basically where there is an "L", next to the electronics. I've found they run way more efficient afterwards. Not sure if the better units have all of this done. Would be a 25 cent fix for the factory to do this. Kind of a bitch when you have to do it yourself.