I've found you need to allow at least two hours for banana slices to freeze in the freezer. Here's how to freeze bananas for nice cream: place banana slices on a tray and place that tray in the freezer. Then transfer to a freezer bag after an hour, that way the slices won't get stuck to each other. Or you can place banana slices directly into a freezer bag, just try to lay the bag flat so the bananas freeze in an individual layer.