We grew zucchini for market for years. Now retired we have trouble with the fruit. They grow to about 5-8 cm when the flower falls off, but then the tip of the fruit goes yellow with eventually the whole fruit going yellow and rotten. It's very frustrating.
Certified GKH Gardening Expert
It sounds like a pollination problem. These articles will help:
You could also be having a problem with blossom end rot. This article will help if you are seeing any of this as well: https://www.gardeningknowhow.com/edible/vegetables/squash/squash-blossom-end-rot-causes-and-treatment.htm