If the zucchini is provided with nutrition constantly (water, fertiliser, etc. then it is usually a problem of pollination of the female flowers. The problem occurs more often when it is cold and wet during the blooming period. This also explains why thermophilic species are prone. Species like „Dundoo“ or „black forest“ are less sensible. You can encounter the problem with hand pollination. You pick a male flower and strip of their pollen directly on the stigma of a female flower. Make also sure not to let the fruits ripen to much as this might also hinder the plant to produce sufficient male and female blossoms.