What's wrong with my grapes?

[thinman56]

I have Frontenac and Frontenac Gris planted at my camp in Bethel, Maine. They're about eleven years old and have been bearing fruit for the last six years. I only get up there to tend to them every two or three weeks, so the greenery gets unruly, but for the most part the vines are healthy. One problem is the clusters are very 'loose' (see pics), so I don't get the yield I expected. Out of 26 vines I get maybe 60 pounds of grapes. Could this be a nutrient issue during cluster formation? Too much leaf left on early in the season?

Thanks for any feedback.

 

[VinesnBines]

The clusters look normal to me. I think both Frontenoc and Frontenoc Gris have fairly loose clusters. These pictures look like loose clusters. Frontenac Frontenac Gris
I don't have either variety but you might research if there is too much vigor or how you are pruning. Maybe they are low yielding. Could you be losing bloom to frost and only getting the secondary crop? I expect the late frost or freezes may be cutting your yield.