I have a 4 door drink cooler that will cool down to about 45 degrees. It will not get any colder, I noticed after having worked on several times that the bottom 1/4 of evaporator coil is all that gets cold, almost immediately starts to frost lines getting cold and eventually freezes only this section of evaporator coil. I have checked filter drier, see no signs of frost on cap tube from where it feeds into the evaporator to where it comes off drier. No signs of restriction but high superheat. Suction pressure running 22psi, head pressure running 215psi, box temperature is at 50 degrees and ambient temp is 88 degrees.

You have a refrigerant leak, most likely. You need to unplug it and pressurize it and look for the leak. Look for oil leaked out of a crack in the tubing and use soap bubbles to find it, if possible. If you still cannot find it you will have to call someone who has something like a D-Tek Leak Detector, the best made. You will have to fix the leak, change the drier, vacuum it down, and re-charge it with whatever refrigerant it uses.