What is wrong with the fridge not cooling?

My refrigerator is warm on the top and bottom, I unplugged it for a few hours to do a reset but it didn't work. I opened the back cover of the freezer to see what is wrong. Everything looks good but after plugging it in to try to see what's wrong I notice that the evaporator starts to frost but after a few seconds it defrosts.

The compressor, condenser fan, condenser coils, relay, capacitor, temperature control thermostat, defrost bi-metal thermostat working so what is wrong?