If this question was directed to me, I have 13 apple trees, 5 of which are mature -- the Cortland is in this group along with its next-tree neighboring MacIntosh -- and 8 trees in a 2 y.o. cider orchard. Last year the fruit crop on the Cortland literally stopped traffic, so I think it took a year off as a result. Not too worried, just found it odd in production.
The trees that are the real iffy ones are the apricots. It seems they bloom too early and then get blasted by a late freeze or they have an off year. The community here said that about once every five years is a good crop.... this last year was it! :) Squirrels got about 40% of my crop, but I still got a pantry full of cots AND gave away flats to the Sr. Center's lunch program, neighbors, coffee klatsch ladies, etc. Really an obscene amount of apricots (and peaches, too!)