Got this from a service agent
The rear wall panel of the fridge covers with frost when the compressor is running, and defrosts when the compressor is off.
If your temperature is set to low against the ambient temperature of the room then the compressor runs constantly or starts up to soon.
This then does not give the defrosting cycle enough time to work and ice will slowly build up.
Defrost completely then set to a higher temperature until the problem stops.
The ambient temperature of all rooms/kitchens are not all the same so the temperature of the fridge compartment has to be set for your circumstances
James Hulse
June 2010