Door Not Closing
The first thing to check when you notice that your freezer is not cold is the door. When you close the door, make sure it is sealing properly. If it feels as if the magnetic seal is closed properly, inspect the gasket for tears. If the gasket is torn, it will allow air to escape, even when you think the door is closed.
Thermostat Not Set Properly
When you are placing food in the freezer, it is often possible to bump the thermostat that regulates the temperature. Look at the thermostat knob, and make sure it is set where it needs to be. Make this easier for yourself by using a permanent marker to note where your unit needs to be set for proper cooling.
Lack of Air Circulation
In order for everything in your freezer to stay at a safe temperature, there needs to be room for air to circulate within the unit. If you find yourself having to approach putting food in your freezer as if it were a jigsaw puzzle, there is a good chance the food will not properly freeze. It will feel cold to the touch, but it will be soft.
Move the refrigerator or freezer so you can see the coils on the back of the unit. If there is dust building up on the coils, it will not allow the unit to operate properly. To clean the coils, first use a vacuum cleaner to remove the loose dust, and then wipe them down with warm, soapy water.
If there is frost or ice building up on the inside of your freezer, chances are there is also ice on the evaporator coils. Most modern units are built to defrost without any consumer activity, but over time this system can fail. The most efficient way to verify that this is the cause of your freezer not cooling is to remove any food from the freezer and leave the door open for 24 hours. Doing this will allow the unit to manually defrost. If this resolves the issue, you may want to call a technician to have the self-defrosting system repaired.