Does the evaporator feel icy cold? (be careful of ice burns when you touch it - use a cloth to feel through so that you won't get stuck to it if it is).
If not then perhaps there is a loss of refrigerant in the system or a faulty compressor
If the evaporator is covered in ice because it is not defrosting properly due to a faulty defrost thermostat then the evaporator fan may also be prevented from rotating due to ice build up therefore no air is being drawn across the evaporator fins to be cooled down and then passed throughout the freezer. The set temperature thermostat keeps the compressor going trying to cool the freezer to the desired temperature but it never reaches the set temperature because the air is not cold enough.
You may also wish to check that the evaporator defrost heater is working properly
Here is a link to all the relevant parts in your freezer. The evaporator fan etc is shown in the cabinet diagram and the defrost heater is shown in the System diagram