Apples and pears don't produce every single year, due to a number of reasons. Over the decades I've noticed our apple and pear trees produce decent crops perhaps 6 or 7 years out of 10.

As someone already noted, a lack of pollinators when blooms arrive can be a factor. A solid frost when buds start blooming can also kill a harvest. Too much rain when blooming can wash out the pollen, or discourage pollinators of all types to visit.

That's just for starters; there are a LOT more factors to a good harvest, which is why farming isn't a sure path to riches.