Lot of times, especially when the plants are very young, a cucumber plant will not form enough male flowers to pollinate the female blossoms. The same thing sometimes happens when the bee activity is too low, even when adequate male and female flowers exist. The inadequately pollinated cucumber will be fat on the stem end and skinny on the blossom end. Those fruit will usually just drop and rot. When the plant starts making adequate male flowers, the fruit will develop normally.
For melon and squash family, the male and female flowers are easy to identify. The female has a tiny fruit at the end of the stem. The male is simply a flower at the end of a stem. For male squash flowers, the stem is quite long.
Here is an image of mine for male and female cucumber flowers.
female cucumber bloom:
male cucumber bloom:
Eclectic gardening style, drawing from 45 years of interest and experience. Mostly plant in raised beds and containers primarily using intensive gardening techniques.