Why is there a sheet of ice on the bottom inside of a frost free freezer?

freezerFrost free freezers perform a defrost cycle and any ice is melted and drains into the pan under the unit. If you have an ice sheet then the drain is probably plugged. this and the pan should be checked regularly. You can access the pan by pulling the bottom grille off.

I have now had two refrigerators with bottom roll-out freezers that had the drain hose from the internal defrost water catch pan plugged with lint like debris. They almost certainly came from the factory plugged. A possible alternative is that dust kittens get sucked into the drain tube when the refrigerator/freezer door is opened and the momentary suction pulls air into the drain tube. The internal catch pan/trough will be full of ice also and will take a long thaw time to clear.

My sisters' freezer had the problem and the mfr. admitted to a too small drain opening and provided a fix.