Just after the flower starts to wilt the fruit becomes stunted, withers and rots. Last year I got almost no yield. I've tried calcium additives, insecticides and bloom spray. You are doing everything right. The blame lies with the bees says UC master gardener Janine Jacobs.
Honeybees pollinate squash and zucchini.
If there are not enough bees present in your neighborhood, the flowers don't get pollinated, and the small fruits wither and die. Without bees, the only way to pollinate those flowers is by hand. You can use a small artist's paintbrush to transfer pollen, or you can break off the male flower and roll it into the center of the female flower. Female flowers can be identified by the miniature fruit at the flower base. Flowers open early in the morning and are receptive to pollination for only one day.