Why? Overripe fruit, wilted greens, rubbery carrots, or mealy apples contain very little water, making it much more difficult to extract juice. Overripe apples will produce just an apple puree. From wilted greens you'll be lucky to get even a few drops of juice. And rubbery carrots can actually damage the juicer.
How to choose: Fruits and vegetables should be juiced as soon as possible after harveting or purchasing. For apples, choose firm varieties. Granny Smith are probably the best, followed by Gala, Red Prince, and Red Delicious. Pineapple and summer pitted fruits like peaches and plums are best juiced before they are fully ripe, while they are still firm. You might be able to save wilted greens or carrots by soaking them in cold water for a few hours.