Your drain tube may 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 drain 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. Flushing it out with hot water and clorox may help. Make sure it drains quick enough.

If the defrost wasn't working you wouldn't have the ice in the bottom of the freezer.