Sounds like a sealed system problem.

This will be an expensive repair as you need a pro.
Often it makes more sense to put the dollars towards a new unit.

I am assuming that the compressor runs all the time other than when the unit goes into a defrost cycle.
Also that the compressor ran for at least an hour before you checked it and found frost on just the bottom of the evaporator/freezer.

Below is a link, take a look at the example pictures part way down the page.
http://www.applianceaid.com/ref...ator-not-cold-enough.php