What's happening with my Pecan trees?

I have a pecan tree that"s nearly 100 years old and 2 others about 30 years old. 2 years ago I got 40 nuts from 3 trees. Last year I got (5) 5 gallon buckets, but some of the nuts were wilted/shriveled up. Could someone tell me what is happening with my trees? Thanks Denise

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