Certified GKH Gardening Expert
Have you pruned any? Grapes form on new wood. This article should help: https://www.gardeningknowhow.com/edible/fruits/grapes/prune-grape-vine.htm
Your soil could also be lacking phosphorus, which is responsible for flowering/fruiting in plants. A soil test will allow you to know for certain; however, it will not hurt to add some bone meal to the soil around your grapevines to add more phosphorus.