Go to this web site and look in the manual on page 15. Check "Compressor runs but will not refrigerate". Gives some suggestions.
Now, inside the evap, usually strapped or clipped to the coil "U" turn on the end of the coil, is a Klixon device. This device comes in many different configurations. There may be 1 or 2 of these. One is a defrost termination switch. When evap gets hot enough so no frost is present, it terminates the defrost cycle. The other is a fan delay. It comes on when the coil temp gets to about 20 degrees. This prevents circulating warm air from defrost cycle. Now if there is only one device, it can be 2 in 1. That is, a defrost terminator/ fan delay. Will most likely have 3 wires going to it where as the 2 separate ones will only have 2 wires to each one. Now, either this fan delay is not working correctly or the coil is not getting cold enough. When it gets cold enough, it turns on the evap fans. This is most likely where your problem lays. Check it out.