I had the same problem. Ice builds up in the drip tray. Answer empty the appliance and let it defrost for 24 hours.
Unplug the appliance and hoover out the dust from the base of the ff. You will have to take off the lower panel at the back after unpluging the appliance. The defrost water runs down a tube and into a drip tray, this tube must be clear otherwise the ice will build up again. I used a plastic tube, and inserted it to the drip tube and blew into it. Some debric did come out. I also filled it with warm water put a finger over both ends then inserted one end into the tube and blew.
Been ok for 2 weeks now.
idlea, March 2012