Adjust the temperature control, if needed. Although this seems simplistic, this can easily be the source of the problem. It should be set at 37 degrees. The temperature control could have been turned up by accident.
Defrost the refrigerator, if necessary. If it has not been defrosted properly, the refrigerator won't cool as well.
Clean the condenser coils with a vacuum. These are behind or beneath the refrigerator.
Inspect the door seal. A worn-out door seal will lead to frost buildup and prevent the door from closing tightly. This results in poor cooling.
Level the refrigerator, if necessary. For the door to shut properly on its own, the refrigerator must be level and slightly tilted back. If it is not level, place a block of wood under the refrigerator and adjust the screws on the leveling legs.
Close the door and make sure it doesn't sag. If the door doesn't shut tightly, it may need adjustment. Use a screwdriver to loosen the hinges and position the door so it is snug. Tighten the hinges.