Your drain tube must be stopped up with ice at the upper end because it drains too slow because it's stopped up at the lower end in the evaporator pan under the unit at the floor. It can get dust and mold in it. Once you get the ice out at the top a little air pressure will usually clear it out. Flush it out with hot water and clorox may help. Make sure it drains quick enough. The drain should be located below the evaporator coils on the lower back of the freezer. | 
Ice Forming in Bottom Freezer 
