I have many flowers but no fruit forming on my cucumbers (or squash). Why?
Cucumbers and squash are by nature dioecious—that is, they have both male and female flowers on the same plant. Typically, there will be more male blooms than females, with the males developing earlier. Until there are also female flowers present, and until conditions favor proper pollination once the boys and girls are both around, you don’t get fruit (which would form right behind the female flower, below, with the male flowers dropping off after providing pollen).
So when we start thinking we’re not getting any fruit despite all the flowers it’s usually either that all the blooms are still male, or that weather conditions are preventing pollination. Unfavorable factors that prevent bees from doing their job include wet or cold weather, or anything (like chemical use) that would eliminate bees, of course.
An exception: Some hybrids have been bred to be gynoecious, or bearing all-female flowers. These require a nearby plant with male flowers to provide pollen, so seed packets of gynoecious cukes typically have some traditional monoecious seeds in them, too (they are usually dyed to tell them apart).