Q.Why Can’t I Get a Zucchini to Grow?
Plant is large, looks healthy, blossoms galore, zucchini start to about 3 inches, then shrivel. Growing organically, treated for white flies, aphids, no visible creatures; bees and butterflies in my garden.

Certified GKH Gardening Expert
It sounds like a pollination problem. Even if you see pollinators in the area, wtih the heat as it's been in much of the country and heavy rains in some areas, pollination becomes more difficult. When this happens, it usually helps to hand pollinate plants.These articles will help:
https://www.gardeningknowhow.com/garden-how-to/beneficial/insect-pollination-process.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/squash-fruit-falling-off-the-plant.htm
https://www.gardeningknowhow.com/edible/vegetables/squash/pollinate-squash-by-hand.htm