I have had 5 grapevines in my yard for many years. They appear to be healthy and grow rapidly in the spring. I do cut them way back in the fall.
In the spring, I get many newly formed clusters on the vines. However, the majority of the clusters turn purple before the grape is soft and then the clusters turn black, hard and dried up. Those clusters that do yield grapes produce delicious grapes. Why do the majority of the clusters not mature into edible grapes?
Dried up grape cluster can be caused by fungal disease. Keep the leaves dry, and the plant pruned to provide good airflow. Use a fungicide labeled for grapes as label directs, as a preventative measure to stop fungus from attacking the grape clusters.