Just a guess:
Maybe you have too many clusters on the vines? Or too many shoots, as DaveL suggested. So the vine doesn't have enough energy to get all the berries "through"?
Easy solution for this problem:
Just cut the green part of the clusters away. As early as possible, so the vine doesn't need to support the useless berries that are not going to ripe anyways and can put all it's energy into the remaining ones!