Answer: Cortland is referred to as self-fruitful but I've found that they produce larger crops if another variety of apple tree is nearby. If you don't want a third apple tree in your yard, you might consider planting a crabapple tree.
You didn't say whether or not your trees are flowering. If they are producing lots of flowers but few fruits, there's a pollination problem - perhaps cold, wet weather which would keep insect activity low. If they are not producing lots of flowers, make sure they are getting all day sunshine and plenty of water during the summer months. Pruning can encourage new flowering growth, but late pruning can reduce your tree's potential to produce flowers. To avoid accidentally cutting off flowering wood, prune your trees in late winter, before new growth begins.
Aside from the above, I can't think of any other reason why your trees are not producing. Try planting a second variety of apple or a crabapple tree and I'll bet you'll see higher fruit production.
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