Poor cooling is often the result of a heavy frost build-up on the evaporator coils or a condenser that is clogged with dust, lint, and dirt.
Poor cooling is often the result of a heavy frost build-up on the evaporator coils. You can't see these coils without removing a panel on the inside of your freezer. A sure sign that there is a build-up is the presence of any frost or ice build-up on the inside walls, floor, or ceiling of the freezer. Such a frost build-up usually indicates a problem in the self-defrosting system or damaged door gaskets.
The refrigerator is supposed to self-defrost approximately four times in every 24 hour period. If one of the components in the self-defrosting system fails, the refrigerator continues to try to cool. Eventually, though, so much frost builds up on the evaporator coils that the circulating fan can't draw air over the coils. There may still be a small amount of cooling because the coils are icy, but with no air flow over the coils, cooling in the refrigerator compartment is quite limited.
Here's an inexpensive, though inconvenient, way to determine if the problem is with the self-defrosting system. Remove all of the perishable food from the refrigerator and freezer, turn the thermostat in the refrigerator to Off, and leave the doors open for 24 to 48 hours. (Be sure to have several towels ready in case the melting frost and ice causes the drip pan to overflow). This allows the refrigerator to defrost "manually." When the frost and ice build-up has completely melted away, turn the thermostat back to a normal setting. If the refrigerator then cools properly, it indicates a problem with one of three components in the self-defrosting system:
The defrost timer
The defrost thermostat (also called the bi-metal switch)
If it still does not cool properly, there may be a problem with the refrigerant level or the compressor. You may need to consult with a qualified appliance repair technician to further diagnose the problem
Self-defrosting refrigerators all have a set of coils and a cooling fan, usually under the refrigerator, that need to be cleaned regularly. If these coils get coated with dust, dirt or lint, the refrigerator may not cool properly. The coils may appear to be a thin, black, wide radiator-like device behind the lower kick-panel. To clean them, disconnect the refrigerator from the power source, use a refrigerator condenser brush and your vacuum cleaner to clean the coils of any lint, pet hair, etc. You may not be able to get to all of the condenser from the front, it may be necessary to clean the remainder of the condenser from the rear of the refrigerator.
You didn’t say what brand/model you have, but check out the LG fridge not cooling page for more ideas—most of them are not brand-specific at all, so they may apply to your situation.