Often the problem is down to clogged condenser coils. Condenser coils carry hot gas and are key to keeping a fridge working properly. But sometimes the coils get clogged and the gas doesn’t travel through properly, stopping the fridge from cooling properly.
Another factor is air circulation. If there isn’t enough air circulation around the unit this could also lead to overheating.
Door seals can also wear and tear with time and overuse, allowing cool air to escape. Damaged seals will also stop your refrigerator or freezer from keeping cool.