The condenser fan motor may not be working or the condenser is dirty. Remove the cardboard at the bottom of the back of the refrigerator. You will see a fan blade. The fan blade should be turning when the compressor is on. If the fan is working make sure to check the condenser coil for dirt. The coil is where the refrigerant expels the heat removed from the inside of the refrigerator. The best was to clean the condenser is using a vacuum to blow air on it. This is messy but works better than just vacuuming it. The other problem could be that the evaporator inside the refrigerator may not be defrosting. The defrost timer may not be working or the defrost heater may be burned out. Another thing I have seen is that the drain which allows the water from the defrost cycle to exit the refrigerator is blocked. This usually has to be cleaned by removing panels from the inside of the freezer. Most of these things can be checked with a volt meter or amprobe if you are handy. Good luck.