Answer: You've covered all the bases and you have me stumped! For proper fruit set, the pollen must be transferred from the male to the female blooms. Anything that interferes with pollination of the female flower (including cold temperatures and rainy weather that hamper bee activity or improper use of insecticides that kill bees) reduces fruit set and yields. With your experience I'm sure you know that the first flowers cucumber plants set are male (with no possibility of setting fruit). As the plant matures, it also sets female flowers (those with a slight swelling directly behind the petals). If they were my plants, I'd first check to see that both male and female flowers are being produced, and if so I'd remove a male flower and brush the insides of the flower into the insides of a female flower. This will transfer pollen and should result in fruit production. Give it a try!
Best wishes with your cucumbers!
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