Hi Bert. It sounds as if they are not being pollinated. Do you have male flowers as well as female? If so you can break off the male flower, peel off the petals, then use the pollen-covered bit to pollinate any open female flowers. Often at the beginning of the season there are not male and female flowers open at the same time so some are not pollinated. Also inside a polytunnel there might not be enough bees to do the pollination for you. Well done though to have flowers at all - mine have none yet, but then I am behind with most things this year.