The fruit is either not getting properly pollinated or it has a disease called botrytis (gray mold). Do you see bees fling in and out of flowers? If no then poor pollination may be the problem. If you are getting a lot of rain or are over head watering so fruit gets wet frequently the the disease may be the problem. If poor pollination, you can try using a small artist paint brush to take pollen from one flower to another. If too wet - try and increase wind and sun light and only water the soil around the plant so the plant stays dry.