A couple things come to mind. In a high-chill year, especially with a particularly warm or cool bloom season, you can have the pollinizer variety come too early or late for the one that most often pollinates it, since they have different chill and heat unit requirements. With high-chill years the bloom period will shorten, sometimes substantially, which can cause seed viability to end before the pollen tube finishes it’s growth; this is variety dependent. Also, what we might consider “good bloom weather” in the spring with high bee activity may actually be too hot; I have seen research that over 80 degrees F (26 C) the flowers can dry out and no longer be viable. Other issues might be the previous year’s crop (too heavy), or strange weather during the bloom initiation the previous year (for either the main variety, or the pollinizer) which could change it’s pollen or flower viability. Hope that helps.