Sounds like a cucumber borer. The larvae of a small brown moth lays an egg near the bloom that then hatches a caterpillar creature that bores into the fruit. Try covering the plants with agricultural fleece to keep the moth from getting to your blooms. You may need to uncover the plants for a few hours in the afternoon when the yellow flowers appear so that they can be pollinated. Or just pollinate the blooms yourself using a paint brush to move pollen from one flower to another then recover the plants to keep off the moths. Often as the season changes so does the insect problem as many moths have a life cycle that ends after a month or so. Maybe later in the season your cucumbers will not have this problem so don't give up. Keep growing, Marianne Binetti